If you own a business — even a business of one — the IRS gives you three retirement-plan choices that don't exist for regular W-2 employees: the SIMPLE IRA, the SEP-IRA, and the Solo 401(k). The three sound like variants of the same idea. They aren't. They're built for three different sizes of business, use three different contribution formulas, live under three different sections of the Internal Revenue Code, and can shelter wildly different amounts of income for the same person earning the same money.[1]
For 2026, the delta between the best and worst choice is not marginal. On the same $50,000 of self-employment profit, a Solo 401(k) shelters $33,794. A SIMPLE IRA under the SECURE 2.0 §117 enhanced limit shelters $19,600. A SEP-IRA shelters $9,294. At $180,000 the spread is wider: Solo 401(k) at $65,957, SIMPLE at $22,400, SEP at $33,457. And at $400,000 all three max out somewhere, but only one of them lets you keep the door open for a clean backdoor Roth. Choosing wrong costs you a five-figure tax bill every year of your working life.[1][9]
This is the guide we wish every small-business owner and freelancer got handed on the day they first started thinking about retirement. It covers the 2026 IRS numbers straight from Notice 2025-67, the algebra behind each plan's contribution formula, the headcount thresholds where a Solo 401(k) becomes a regular 401(k) and where a SIMPLE IRA is forced to become a Safe Harbor 401(k), three worked case studies at typical income levels, the deadline calendar for opening and funding each account, the Roth-availability landscape under SECURE 2.0, and the backdoor-Roth pro-rata trap that quietly disqualifies both SEP and SIMPLE holders. When you're ready to run your own numbers, our SIMPLE IRA calculator, SEP-IRA contribution calculator, 401(k) calculator, and self-employment tax calculator handle the arithmetic.
🏦Compare all three plans on your income in 60 seconds
Free calculator — enter W-2 wages or Schedule C net, see the 2026 maximum for SIMPLE, SEP, and Solo 401(k) side by side.
The three plans in one sentence each
Before we drown you in IRS notice numbers, here is the shortest possible version of the choice.
SIMPLE IRA — Savings Incentive Match Plan for Employees. A small-employer (≤100 employees) plan that requires the employer to make a match or nonelective contribution every year. Employees contribute up to $17,000 in 2026 as an elective deferral; the employer adds a 3% match or 2% nonelective on top. Simple to administer, mandatory for all eligible employees, and every dollar has to come out of the same employer's pocket for everyone. Governed by IRC §408(p).[3]
SEP-IRA — Simplified Employee Pension. A discretionary employer plan where only the employer contributes (up to 25% of compensation), the same percentage for every eligible employee including the owner. Zero elective deferrals from the employee side. Easiest to establish (a single-page Form 5305-SEP), best deadline flexibility (funded through the extended tax return due date), and the plan the IRS specifically designed for businesses that want a "set it and forget it" retirement vehicle. Governed by IRC §408(k).[4]
Solo 401(k) — one-participant 401(k). A full qualified retirement plan under IRC §401(a) for a business with zero common-law employees other than the owner and the owner's spouse. Both employee elective deferral ($24,500 in 2026) AND employer profit-sharing (25% / 20%) buckets are available and they stack — up to a §415(c) $72,000 ceiling in 2026. Highest shelter of the three by a mile, but the moment you hire a common-law employee who crosses coverage thresholds, the Solo 401(k) becomes a regular 401(k) with all the compliance baggage that entails.[2]
The pick-in-30-seconds rule
No common-law employees other than you (and maybe your spouse) → Solo 401(k). Small team of 1–5 employees you want to reward with matching contributions → SIMPLE IRA (or a Safe Harbor 401(k) if the numbers justify it). Team of 6–100 employees where you want the option to skip contributions in bad years → SEP-IRA. Over 100 employees → SEPs and SIMPLEs are both off the table; you're in Safe Harbor 401(k) territory.
The 2026 contribution limits, all three plans side by side
The IRS published the 2026 cost-of-living adjustments in Notice 2025-67 on October 24, 2025, formalized through News Release IR-2025-176 the same day.[1][2] The single reference table:
| Provision | SIMPLE IRA | SEP-IRA | Solo 401(k) | Statute |
|---|---|---|---|---|
| Employee elective deferral (base) | $17,000 | None | $24,500 | IRC §408(p)(2)(E), §402(g) |
| Enhanced elective deferral (SECURE 2.0 §117) | $18,100 | — | — | IRC §408(p)(2)(E)(ii) |
| Employer contribution | 3% match OR 2% nonelective | ≤ 25% of comp (20% of SE net) | ≤ 25% of comp (20% of SE net) | IRC §408(p)(2), §404(h), §404(a)(3) |
| Overall §415(c) DC limit | N/A (uncapped by §415(c)) | $72,000 | $72,000 | IRC §415(c)(1)(A) |
| Age 50+ catch-up (base) | +$4,000 | None | +$8,000 | IRC §414(v) |
| Age 50+ catch-up (SECURE 2.0 §117 enhanced) | +$4,850 | — | — | IRC §414(v)(2)(E) |
| Age 60–63 super catch-up (SECURE 2.0 §109) | +$5,250 | None | +$11,250 (in lieu of $8,000) | IRC §414(v)(2)(E) |
| §401(a)(17) compensation cap | $360,000 | $360,000 | $360,000 | IRC §401(a)(17) |
| Max headcount | ≤ 100 employees | Unlimited | Owner + spouse only | IRC §408(p)(2)(C), §408(k), §401(k)(2)(D) |
| Roth option | Yes (SECURE 2.0 §601, 2023+) | Yes (SECURE 2.0 §601, 2023+) | Yes since 2006 (IRC §402A) + Roth employer 2023 | IRC §408A(f), §402A, §402A(a)(1) |
| Backdoor Roth pro-rata aggregation | Aggregates with all IRAs | Aggregates with all IRAs | Excluded | IRC §408(d)(2) |
| Loans permitted | No | No | Yes, up to lesser of $50,000 or 50% | IRC §72(p) |
| Form 5500-EZ filing | Never | Never | Once assets > $250,000 | IRC §6058 |
| Setup deadline (plan year 2026) | October 1, 2026 | Tax filing deadline + ext (Oct 15, 2027) | December 31, 2026 (plan) / Oct 15, 2027 (funding) | IRC §408(p)(2)(A)(v), §408(k), SECURE Act §201 |
All figures verified against IRS Notice 2025-67 and IRS "COLA increases for dollar limitations on benefits and contributions" as of publication date.[1][2]
The SECURE 2.0 §117 110% enhancement — why some SIMPLE plans get $18,100
SECURE 2.0 §117 (enacted December 2022, effective 2024+) grants a 110% enhanced elective-deferral limit for SIMPLE IRA plans if the employer has 25 or fewer employees earning at least $5,000 in the prior year. The enhancement is automatic. Employers with 26 to 100 employees can also opt in if they increase their match to 4% (from the standard 3%) or their nonelective to 3% (from the standard 2%). For 2026 the enhanced base deferral is $18,100 (110% × $16,455 rounded per Notice 2025-67 conventions), and the enhanced age-50 catch-up is $4,850. The age 60–63 super catch-up under §109 is $5,250 regardless of enhancement.[3][8]
The employer contribution math — three formulas that all look like percentages but aren't
Each plan's employer bucket uses a percentage in the marketing copy that quietly means something different once you sit down with the plan document.
SIMPLE IRA — either 3% match or 2% nonelective, no middle ground
Under IRC §408(p)(2)(A)(iii) and (p)(2)(B), the employer must choose one of two contribution formulas each year, elected via Form 5304-SIMPLE (employee-directed custodian) or Form 5305-SIMPLE (employer-selected DFI).[6]
Option A: 3% match. The employer contributes a dollar-for-dollar match on employee deferrals, up to 3% of the employee's compensation. If the employee doesn't defer, the employer doesn't match. The employer may reduce the 3% floor to as low as 1% in any two out of any five consecutive years (the "step-down" provision), which is the standard escape hatch for cash-flow-strapped years. An employee earning $80,000 who defers 3% ($2,400) gets a $2,400 match. An employee earning $80,000 who defers 10% ($8,000) still gets only a $2,400 match (capped at 3% of comp).
Option B: 2% nonelective. The employer contributes 2% of each eligible employee's compensation, up to the §401(a)(17) $360,000 comp cap = $7,200 maximum per employee for 2026. Nobody has to defer to trigger the contribution; it goes into every eligible account whether they participate or not. This option is often preferred by employers with low employee-participation rates because it guarantees the contribution shows up in the plan and satisfies the SIMPLE mandate regardless of employee behavior.
SECURE 2.0 §116 additional 10% nonelective (2024+). On top of either Option A or Option B, employers may make an additional nonelective contribution of up to 10% of compensation, capped at $5,000 per employee for 2026 (indexed).[8] The cap must apply to all eligible employees uniformly. This provision was written for employers who want to boost contributions without switching to a Safe Harbor 401(k) — it takes the total employer commitment from 3% to potentially 13% of compensation for the higher-earning eligible participants.
SEP-IRA — 25% of comp for W-2, 20% of net for Schedule C, same for everyone
The SEP-IRA has one contribution formula and it applies to every eligible employee (owner included) at the same rate. Under IRC §404(h), the employer contributes up to 25% of an employee's IRC §401(c)(2) compensation, capped by the §415(c) $72,000 ceiling and the §401(a)(17) $360,000 comp cap.[4]
For a W-2 employee (including an S-corp shareholder-employee), 25% of comp is straightforward. For a self-employed sole proprietor, §401(c)(2) defines compensation as net earnings from SE minus ½ SE tax minus the contribution itself. The circular definition (contribution reduces comp, comp defines contribution) resolves algebraically:
C + 0.25C = 0.25 × (NE − ½SE)
1.25C = 0.25 × (NE − ½SE)
C = 0.20 × (NE − ½SE)
The 25% on the plan document becomes an effective 20% of net-earnings-after-half-SE-tax when applied to a Schedule C sole proprietor. The IRS calls this the "reduced rate" and publishes a Deduction Worksheet for Self-Employed in Publication 560, Chapter 5.[4] Miss the adjustment and contribute a straight 25% of Schedule C profit and you'll trigger a 6% annual excise tax under IRC §4973 for the excess portion.
The equal-percentage trap that kills SEP-IRAs for growing businesses
Every eligible employee must receive the same contribution rate. If you (the owner) want a 20% contribution for yourself, you must contribute 20% of comp for every eligible employee. On a payroll of five employees averaging $60,000, that's $60,000 of employer commitment before you fund your own account. This is why SEP-IRAs quiet die as businesses grow past 3–4 employees — the same-rate mandate makes them dramatically more expensive than a SIMPLE (3% match) or a Safe Harbor 401(k) (3% nonelective) at similar shelter levels for the owner.
Solo 401(k) — both buckets, both math styles, stacked to $72,000
The Solo 401(k) combines the two other plans' contribution styles. The employee elective deferral is a flat dollar amount ($24,500 in 2026, plus catch-ups) under IRC §402(g). The employer profit-sharing contribution follows the exact same 25% / 20% formula as the SEP-IRA under IRC §404(a)(3). Both buckets aggregate against the §415(c) $72,000 ceiling.[1][2]
total ≤ $72,000 (plus age catch-ups)
The elective deferral does not require a percentage of compensation — it can be 100% of comp up to $24,500. This is why a freelancer with $30,000 of Schedule C profit can shelter roughly $24,500 in a Solo 401(k) but only about $5,575 in a SEP-IRA. It's also why the Solo 401(k) is disproportionately valuable for low-to-moderate income self-employed people — the flat $24,500 dwarfs the percentage-based employer bucket at those income levels.
🧾Compute your SE tax first, then your contribution
Half SE tax is the input to both the SEP and Solo 401(k) profit-sharing formula. Get it right, then feed it into whichever plan you pick.
Three case studies at $50K, $180K, and $400K
Numbers make the trade-off concrete. Here are three real-world profiles worked end to end.
Case 1: Jasmine, $50,000 Schedule C, age 34, no employees
Jasmine is a solo web-development freelancer. No S-corp election, no employees, no spouse in the business.
- Net earnings for SE = $50,000 × 0.9235 = $46,175
- SE tax = $46,175 × 15.3% = $7,064.78; deductible half = $3,532.39
- Net-earnings-after-half-SE = $50,000 − $3,532.39 = $46,467.61
- Employer contribution (20%): 0.20 × $46,467.61 = $9,293.52
| Plan | Employee deferral | Employer contribution | Total sheltered | vs Solo 401(k) |
|---|---|---|---|---|
| SIMPLE IRA (base, 3% match) | $17,000 | $1,500 (3% × $50K) | $18,500 | −$15,294 |
| SIMPLE IRA (§117 enhanced, 3% match) | $18,100 | $1,500 | $19,600 | −$14,194 |
| SEP-IRA | — | $9,294 | $9,294 | −$24,500 |
| Solo 401(k) | $24,500 | $9,294 | $33,794 | — |
Winner: Solo 401(k) by $14,194–$24,500. The flat elective deferral of $24,500 dominates every other structure at this income level. Jasmine has no employees so a Solo 401(k) is trivially available. Over a 31-year working horizon from age 34 to age 65 at a 7% real return, the $14,194 annual difference vs the best SIMPLE alternative compounds to roughly $1.32 million in retirement.[9] One dollar of admin friction (adopting a plan document) buys her the difference between a comfortable retirement and a great one.
Case 2: David, $180,000 Schedule C, age 52, no employees
David is a fractional CFO consultant, age 52, so eligible for the age-50+ catch-up.
- Net earnings for SE = $180,000 × 0.9235 = $166,230
- SS portion (capped at $184,500 wage base 2026) = $166,230 × 12.4% = $20,612.52
- Medicare = $166,230 × 2.9% = $4,820.67
- SE tax = $25,433.19; deductible half = $12,716.60
- Net-earnings-after-half-SE = $167,283.40
- Employer contribution (20%): $33,456.68
| Plan | Employee deferral | Catch-up | Employer contribution | Total sheltered |
|---|---|---|---|---|
| SIMPLE IRA (base) | $17,000 | $4,000 (age 50) | $5,400 (3% × $180K) | $26,400 |
| SIMPLE IRA (§117 enhanced) | $18,100 | $4,850 | $5,400 | $28,350 |
| SEP-IRA | — | None | $33,457 | $33,457 |
| Solo 401(k) | $24,500 | $8,000 (age 50) | $33,457 | $65,957 |
Winner: Solo 401(k) by $32,500. The age-50 catch-up (which SEPs and SIMPLEs both restrict or omit) matters more as income rises. David's marginal federal rate at $180,000 as single filer (TY2025 24% bracket per Rev. Proc. 2024-40) means the extra $32,500 of shelter versus SEP is worth roughly $7,800 in current-year federal tax savings, plus whatever his state costs him.[7] Solo 401(k) also unlocks the loan option ($50,000 max under IRC §72(p)) if he ever needs bridge capital — SEP and SIMPLE both explicitly prohibit loans.
Case 3: Priya's boutique agency, 6 employees, average $65,000 salary
Priya runs a lifestyle-medicine consultancy with 6 W-2 employees averaging $65,000 in wages. Priya pays herself $200,000 W-2 wages from the S-corp. Her Solo 401(k) is off the table (common-law employees crossed the coverage threshold). Her real choice is SIMPLE IRA vs SEP-IRA vs Safe Harbor 401(k). Assume all 6 employees participate and defer at least 3% for the SIMPLE and Safe Harbor math.
| Plan | Priya's shelter | Employer cost (6 employees) | Total plan cost | Compliance |
|---|---|---|---|---|
| SIMPLE IRA (§117 enhanced, 3% match) | $18,100 + $6,000 match = $24,100 | 3% × $390K = $11,700 | $35,800 | No 5500-EZ; no ADP testing |
| SEP-IRA (10% for everyone) | 10% × $200K = $20,000 | 10% × $390K = $39,000 | $59,000 | No 5500; no discrimination testing |
| SEP-IRA (20% for everyone) | 20% × $200K = $40,000 | 20% × $390K = $78,000 | $118,000 | No 5500; no discrimination testing |
| Safe Harbor 401(k) (basic match) | $24,500 EE + $24,500 profit-share + $6,000 SH match (3%) = $54,000+ | 3% SH match × $390K = $11,700 + optional PS | $65,700+ | Form 5500-SF required; TPA cost ~$2,000/yr |
Priya's actual decision is dominated by employer cost per dollar of owner shelter. The SIMPLE gives her $24,100 for a $35,800 total cost — a ratio of 67 cents of shelter per dollar spent. The SEP at 20% gives her $40,000 for $118,000 — 34 cents per dollar. The Safe Harbor 401(k) gives her $54,000+ for $65,700+ — 82 cents per dollar and the best absolute shelter, but she pays a TPA and files Form 5500 annually. The correct answer for most 6-employee businesses at this scale is either the SIMPLE (if cash flow is tight) or the Safe Harbor 401(k) (if the owner wants to max her personal shelter and the extra $12,000 of employer cost is acceptable). The SEP is rarely the right answer once headcount exceeds 3–4 employees.[4][5]
The general result at every income level
For a solo owner with no employees, the Solo 401(k) always wins on shelter size — the $24,500 elective deferral is a fixed advantage that no other plan can match. For a small team (2–10 employees), the SIMPLE IRA usually wins on shelter-per-employer-dollar, and the SEP is competitive only when the owner is willing to fund every employee at the same 20%+ rate. For a mid-sized team (10–100 employees), the Safe Harbor 401(k) becomes viable because the TPA cost amortizes and the ADP/ACP nondiscrimination-testing exemption starts mattering. Over 100 employees, SIMPLE is disqualified and full 401(k) is your only path.
The deadline calendar — miss it and the plan doesn't exist for that year
Each plan has a different "must exist by" date and different funding deadlines. Get the setup deadline wrong and the entire tax year is lost.
| Plan | Plan-establishment deadline | Employer-contribution deadline | Employee-deferral deadline |
|---|---|---|---|
| SIMPLE IRA | October 1 of the plan year (IRC §408(p)(2)(A)(v)) | Tax filing deadline + extensions (April 15 / October 15) | Within 30 days after month of deferral (DOL rules) |
| SEP-IRA | Tax filing deadline + extensions (October 15) | Same — tax filing deadline + extensions | N/A (no elective deferrals) |
| Solo 401(k) — deferrals | December 31 of the plan year (plan document + written salary-deferral election) | — | Cash landed within reasonable time; election made by year-end |
| Solo 401(k) — profit-sharing | Same December 31 deadline for plan; contribution funded by tax filing + extensions per SECURE Act §201 | Tax filing deadline + extensions (October 15) | — |
The most common failure mode: a solo consultant realizes in February 2027 that she should have set up a Solo 401(k) for 2026 tax purposes. She's out of luck — the plan document had to exist by December 31, 2026 for elective deferrals to count for 2026. Her only option is to set up a SEP-IRA (which she can still establish and fund through October 15, 2027 with a Form 4868 extension) and take the smaller shelter. This is the single most compelling reason to open a Solo 401(k) plan document in October–November of the current year, even if you're not 100% sure you'll contribute — the plan document is free at Fidelity, Schwab, Vanguard, and E*TRADE, and having the plan in place preserves the option.[2][11]
The SIMPLE IRA October 1 deadline is a hard wall
Unlike the Solo 401(k) and SEP-IRA deadlines that reach into the following calendar year with extensions, the SIMPLE IRA plan must be established no later than October 1 of the plan year it covers, per IRC §408(p)(2)(A)(v). For 2026 coverage, that's October 1, 2026 — a Thursday. There is no extension. The only exception: a business established after October 1 in a given year may establish a SIMPLE as soon as administratively feasible after the business begins.[3] If you miss the deadline, your only path to 2026 shelter is a SEP-IRA. Set a calendar reminder for September 15 every year to force the decision before the wall arrives.
Roth availability under SECURE 2.0
Roth deferrals in all three plans are now legally permitted, but custodian rollout has been staggered.
Solo 401(k). Roth elective deferrals have been permitted since 2006 under IRC §402A. SECURE 2.0 §604 (effective for taxable years after December 29, 2022) added the option for employers — which for a Solo 401(k) means the owner themselves — to elect that some or all of the employer profit-sharing contribution be treated as Roth (rather than pre-tax). Every mainstream custodian's Solo 401(k) plan document has been updated to reflect this option.[8]
SEP-IRA. SECURE 2.0 §601 authorized Roth SEP contributions effective for tax years after December 31, 2022. Custodian rollout has been slow — Fidelity added Roth SEP in mid-2024, Vanguard in early 2025, Schwab in mid-2025. Confirm your custodian supports it before electing Roth, or open a separate Traditional SEP alongside your Roth SEP if the mainline custodian is behind.[3]
SIMPLE IRA. SECURE 2.0 §601 also authorized Roth SIMPLE contributions effective for tax years after December 31, 2022. Custodian rollout has been comparable to Roth SEP — most mainstream custodians rolled out by mid-2025.[8]
SECURE 2.0 §603 mandatory Roth catch-up. For 401(k) and 403(b) plans (including Solo 401(k)), participants whose prior-year FICA wages exceeded $150,000 (2026 threshold indexed from the $145,000 statutory base) must designate their catch-up contributions as Roth. Solo 401(k) owners taking Schedule C income are exempt — sole proprietors don't receive FICA wages, so the rule doesn't reach them. S-corp shareholder-employees using a Solo 401(k) with W-2 wages above $150,000 ARE subject, and must designate their $8,000 age-50 catch-up (or $11,250 age-60–63) as Roth. This rule does not apply to SIMPLE IRA or SEP because those plans have different catch-up mechanics under IRC §414(v)(2).[8]
Roth SEP and Roth SIMPLE — the pro-rata trap gets weirder
Roth contributions to a SEP or SIMPLE IRA do NOT clean up the §408(d)(2) pro-rata problem. Both accounts are still legally IRAs, and the aggregation for backdoor Roth conversion pro-rata under §408(d)(2) sums all Traditional, SEP, SIMPLE, and Rollover IRA balances. If your SEP has $50,000 of Roth contributions and $50,000 of pre-tax employer contributions, only the $50,000 pre-tax portion counts in the numerator of the pro-rata calculation, but it still contaminates every backdoor Roth conversion for as long as it exists. The clean-up is either a rollover of the pre-tax portion into an accepting Solo 401(k), or a very-large Roth conversion of the pre-tax portion in a single tax year to zero out the pro-rata trap.
The backdoor Roth pro-rata trap — and why it matters for two of the three plans
Under IRC §408(d)(2), all Traditional, SEP, SIMPLE, and Rollover IRA balances aggregate for the purpose of computing the taxable portion of any Roth conversion. Qualified plan balances under IRC §401(a) — including Solo 401(k)s — are excluded from the aggregation.[4]
Practically: if you have a $30,000 SIMPLE IRA balance and try to execute a $7,500 backdoor Roth via a fresh nondeductible Traditional IRA contribution followed immediately by a conversion to Roth, the taxable portion of the conversion is:
Taxable portion = $7,500 × 0.80 = $6,000
You just paid federal tax on $6,000 of what you thought was a clean backdoor Roth conversion of $7,500 of nondeductible contributions. At a 24% marginal rate that's $1,440 of unexpected federal tax, plus state. For high earners deploying the backdoor Roth every year, this trap represents thousands of dollars of leakage annually and it doesn't go away — it recurs every conversion year until the pre-tax IRA balance is zero.
The defusing options for someone with a contaminated SIMPLE or SEP:
- Roll the pre-tax IRA balance into a Solo 401(k) that accepts rollovers-in. Not all Solo 401(k) plan documents accept rollovers — verify before electing. Once the SIMPLE or SEP balance is inside a qualified plan under §401(a), it's out of the §408(d)(2) aggregation.
- Convert the entire pre-tax IRA balance to Roth in one shot. Expensive in the current year (pay ordinary-income tax on the full pre-tax balance), but permanently defuses the trap. Best done in a low-income year (job transition, sabbatical, business loss).
- Wait out the SIMPLE 2-year window before rolling. Under IRC §72(t)(6), SIMPLE IRAs have a 2-year restriction on rollovers out to non-SIMPLE plans. Roll to a SIMPLE-eligible destination during the first 2 years or wait past the anniversary before rolling into a Solo 401(k).
See our companion piece on the Backdoor Roth IRA in 2026 for the complete pro-rata defusing playbook and Form 8606 line-by-line walkthrough. The trap is real, the fix is straightforward once you know it exists, and the money at stake for high-income savers is a five-figure recurring annual cost.
🔄Model your Roth conversion with pro-rata
Our Roth conversion calculator handles the §408(d)(2) pro-rata math with your SEP or SIMPLE balance as an input.
Headcount triggers — when each plan is forced out
Each plan has a maximum-employee threshold above which it stops working. The transitions are painful because they force a plan-type change with corresponding admin friction.
Solo 401(k) → regular 401(k) at "any common-law employee"
The moment your business hires a common-law employee who satisfies the coverage rules of IRC §410(a) — one year of service with 1,000+ hours, OR (post-SECURE 2.0 §125, effective 2025) three consecutive years with 500+ hours, whichever comes first — the Solo 401(k) becomes a regular 401(k). ADP/ACP nondiscrimination testing kicks in under IRC §401(k)(3) and (m)(2). Form 5500 (not the -EZ variant) is required. The plan sponsor typically needs a Third-Party Administrator to handle the compliance work.[2]
The main escapes: (a) hire only part-time employees under 500 hours/year across every 12-month period; (b) hire only your spouse (spouse exception preserves Solo 401(k) status regardless of hours); (c) hire independent contractors (1099) instead of W-2 employees — but the classification has to be defensible under the IRS 20-factor test.[14]
SIMPLE IRA → not eligible at 101+ employees
Under IRC §408(p)(2)(C)(i), an eligible employer for a SIMPLE IRA is one that "employed no more than 100 employees who received at least $5,000 in compensation" during the prior calendar year.[3] Once the 101st eligible employee is on payroll during a prior year, the SIMPLE cannot be established for the following year, and existing SIMPLEs must terminate within a grace period defined in the SIMPLE plan rules (generally a 2-year grace period after crossing the threshold).
The typical transition path: SIMPLE at ≤100 employees → Safe Harbor 401(k) at 100+ employees. Safe Harbor requires either a 3% nonelective to all eligible employees or a basic match (100% on first 3%, 50% on next 2%) that qualifies for the ADP/ACP nondiscrimination-testing exemption under IRC §401(k)(12). SECURE 2.0 §332 (effective 2024) provides a mid-year SIMPLE-to-safe-harbor transition provision that eases the timing pain.[8]
SEP-IRA → still functional but usually not desirable at 4+ employees
The SEP-IRA has no hard headcount cap — legally you can run a SEP with 500 employees. But the equal-contribution-rate mandate under §408(k)(3)(D) becomes prohibitively expensive: if the owner wants 20% for herself, she must fund 20% of comp for every eligible employee. Most SEP plans that grow past 3–4 employees terminate in favor of a Safe Harbor 401(k) or a SIMPLE IRA. The employees-must-be-covered rule kicks in for any employee earning $750+ in the current year who has worked for the employer in 3 of the immediately preceding 5 years and is at least age 21 (per IRC §408(k)(2), as adjusted by SECURE Act 2.0 §125).[4]
The headcount ladder for retirement plans
0 employees (owner + spouse): Solo 401(k). 1–5 employees: SIMPLE IRA. 6–100 employees: SIMPLE IRA (if you're OK with the elective-deferral cap) OR Safe Harbor 401(k) (if you want higher owner shelter). 101–500 employees: Safe Harbor 401(k). 500+ employees: Cash Balance / Defined Benefit hybrid layered on top of the 401(k) for very high owner shelter.
Administrative friction and ongoing filings
The three plans differ dramatically in what you have to do to keep them alive.
| Requirement | SIMPLE IRA | SEP-IRA | Solo 401(k) |
|---|---|---|---|
| Plan document | Form 5304-SIMPLE or 5305-SIMPLE (IRS template) | Form 5305-SEP (IRS template) or custom prototype | Custodian prototype (Fidelity / Schwab / E*TRADE free) |
| Annual notice to employees | Yes, 60 days before election period | No | Summary Plan Description if participants exist |
| Form 5500 filing | Never (exempt under §6058) | Never (exempt under §6058) | Form 5500-EZ once assets > $250,000 |
| Nondiscrimination testing | None | None (uniform-rate mandate obviates) | None (single participant) |
| Loan permissible | No (IRC §72(p) restricts) | No | Yes, up to $50,000 or 50% of vested balance |
| Custodian fees (typical) | $0–$25/participant/year | $0 | $0 at Fidelity/Schwab; $100–$500/yr at Solo-K specialists |
The Solo 401(k) Form 5500-EZ once assets exceed $250,000 is the only meaningful ongoing filing across the three plans. It's a short form (roughly 2 pages) and the plan sponsor files it directly with the IRS by July 31 of the year following the plan year. Miss the filing and the penalty under IRC §6652(e) is $250/day up to a maximum of $150,000 per year — steep enough that the IRS runs a Voluntary Compliance Program for late filers that caps the penalty at $500–$1,500 per return.[11]
Switching between plans — when and how
Businesses evolve. A solo consultant hires her first employee and needs to move from Solo 401(k) to something. A SIMPLE IRA employer decides she'd rather maximize her personal shelter and migrate to Solo 401(k) or Safe Harbor 401(k). The switching rules are strict and generally require January-1 transitions.
SIMPLE IRA → Solo 401(k) or SEP. IRC §408(p)(2)(D) prohibits an employer from maintaining a SIMPLE and any other qualified plan in the same calendar year. The employer must give employees a 60-day notice by November 2 that the SIMPLE will terminate at year-end. On January 1 the replacement plan takes over. SECURE 2.0 §332 created a mid-year replacement provision for SIMPLE-to-Safe-Harbor-401(k) only — SIMPLE-to-Solo still requires the January-1 wall.[8]
SEP-IRA → Solo 401(k) or SIMPLE. No exclusive-plan restriction. The employer can open a Solo 401(k) or SIMPLE for the following year, freeze new contributions to the SEP, and roll the SEP balance into the Solo 401(k) (if the destination plan document accepts rollovers-in) or leave it in place as a legacy account.
Solo 401(k) → regular 401(k) or SIMPLE. When common-law employees cross the coverage threshold, the Solo 401(k) automatically converts by operation of law into a regular 401(k). The plan sponsor has an administrative-cure window (typically the plan year in which the crossing occurs) to either adopt Safe Harbor provisions or terminate the plan and switch to SIMPLE.
Roll-over rules. SIMPLE-to-non-SIMPLE rollovers require satisfaction of the 2-year clock under IRC §72(t)(6). SEP-to-Solo-401(k) rollovers are unrestricted (SEP is functionally an IRA and IRAs accept rollover to qualified plans that accept them). Solo-401(k)-to-IRA rollovers are unrestricted post-termination.[3]
Worst-case mistakes and how to avoid them
The five most common failure modes we see in practice:
1. Contributing exactly 25% of Schedule C profit to a SEP. Should be 20% of net-earnings-after-half-SE-tax per IRC §401(c)(2). Over-contribution triggers 6% annual excise tax under §4973 until corrected. Fix: file Form 5329, withdraw the excess plus earnings before the tax-filing deadline.[4]
2. Missing the October 1 SIMPLE deadline. No extension possible. Fix: switch to SEP-IRA for the current year (SEP deadline extends to October 15 with extensions) and open SIMPLE for next year.
3. Missing the December 31 Solo 401(k) plan-document deadline. The plan must exist by year-end for elective deferrals to count. Fix: file for tax extensions, and open a SEP for the current year (SEP extends to October 15 following); open the Solo 401(k) by December 31 for next year to preserve the option.
4. Executing a backdoor Roth with a SEP or SIMPLE balance in place. Pro-rata trap contaminates the conversion. Fix: roll the SEP/SIMPLE balance into a Solo 401(k) before December 31 of the conversion year, or convert the entire pre-tax balance in one year to defuse permanently.
5. Hiring a common-law employee while running a Solo 401(k) without a plan-type transition plan. Solo 401(k) becomes a regular 401(k) with ADP/ACP testing, Form 5500, and possibly disqualification of prior contributions. Fix: implement Safe Harbor provisions before the coverage-threshold year, OR terminate the Solo 401(k) and open a SIMPLE for the following year.[2]
The Social Security wage base and marginal-rate context for 2026
Retirement-plan contribution decisions live inside a bigger tax picture. For 2026 the Social Security Administration set the Social Security wage base at $184,500, up from $176,100 in 2025.[12] The 12.4% SS portion of SE tax applies only to net earnings for SE up to that ceiling; above it, only the 2.9% Medicare portion (plus the 0.9% Additional Medicare over $200K single / $250K MFJ) applies. This matters for SEP and Solo 401(k) profit-sharing math because the "half SE tax" deduction that enters the §401(c)(2) compensation formula shrinks proportionally once you cross the wage base.
Federal income-tax brackets for 2025 (still applicable through most of 2026 for planning purposes, with 2026 Rev. Proc. 2025-32 numbers due Q4 2026) put single filers into the 24% bracket at $103,350 and MFJ into the 24% bracket at $206,700 (per Rev. Proc. 2024-40 §3.01).[7] The value of an additional $10,000 of shelter at those marginal rates is $2,400 in current-year federal tax, plus state (typically $200–$1,000 depending on jurisdiction). Over a 30-year working horizon, deferring $10,000/year at a 7% real return generates $1.01M of retirement balance vs $0 not deferred — the tax shelter is the smallest part of the win.[9]
Your action checklist
- Count your employees (including yourself). Zero except owner + spouse → Solo 401(k). 1–100 → SIMPLE or Safe Harbor 401(k). 100+ → Safe Harbor 401(k) or full 401(k) only.
- Calculate your Schedule C net or W-2 wages. Below $50K, the Solo 401(k) elective-deferral advantage dominates by 3-4x. Above $200K, all three plans push toward their §415(c) or elective-deferral caps and the choice is about admin friction.
- Check your existing IRA balances. If you have a Traditional, Rollover, SEP, or SIMPLE IRA and want to keep the backdoor Roth door open, plan the pro-rata defusing (rollover into Solo 401(k) or full conversion) before December 31 of the target year.
- Set your calendar. September 15 = decide about SIMPLE for the current year (October 1 wall). October 15 = decide about opening a Solo 401(k) plan document (December 31 wall). April 15 (or extended October 15) = SEP funding deadline.
- Open the plan document. All three plans can be opened for free at Fidelity, Schwab, Vanguard, or E*TRADE. The Solo 401(k) prototype documents at Fidelity and Schwab include the SECURE 2.0 §601 Roth-employer election.
- Elect Roth or pre-tax at the deferral level. Under-40 in the 24%+ bracket and expecting higher rates in retirement → lean Roth. Over-50 already saving heavily → lean pre-tax to reduce current AGI.
- Fund by the deadline. Solo 401(k) elective deferrals must be documented as of December 31 even if cash lands later. SEP and SIMPLE funding can extend to the tax-filing deadline plus extensions.
- File Form 5500-EZ once your Solo 401(k) assets exceed $250,000. Due by July 31 of the following year. Miss it and the penalty starts at $250/day.[11]
Run all three side by side on your income
Enter W-2 wages or Schedule C net and see the exact 2026 shelter under SIMPLE (base + §117 enhanced), SEP, and Solo 401(k).
Frequently asked questions
What is the biggest difference between a SIMPLE IRA, SEP-IRA, and Solo 401(k) in 2026?
The three accounts serve different-sized businesses. A Solo 401(k) is designed for a business with zero common-law employees (owner + spouse only) and shelters up to $72,000 in 2026. A SEP-IRA is a discretionary employer-only plan capped at 25% / 20% and works from zero to any number of employees but requires equal-percentage contributions for everyone. A SIMPLE IRA is a small-employer plan (≤100 employees) with a mandatory employer match or nonelective — the 2026 employee deferral is $17,000, or $18,100 under SECURE 2.0 §117. Rule of thumb: no employees → Solo 401(k); 1–5 employees you want to reward → SIMPLE with match; 6–100 employees where you want maximum employer flexibility → SEP; growing past 100 employees → Safe Harbor 401(k).
What are the 2026 SIMPLE IRA contribution limits?
The 2026 SIMPLE IRA employee elective deferral limit is $17,000 (up from $16,500 in 2025). Under SECURE 2.0 §117, employers with 25 or fewer employees may elect a 110% enhanced limit of $18,100 automatically, and 26–100 employees may opt in with a matching-contribution enhancement. The age 50+ catch-up is $4,000 (standard) or $4,850 under the 110% enhancement. The SECURE 2.0 §109 age 60–63 super catch-up is $5,250. Employer contributions are either a 3% match on 100% of comp or a 2% nonelective to all eligible employees.
Why can't I use a Solo 401(k) if I have even one part-time employee?
A Solo 401(k) is defined as a "one-participant 401(k)" — it stays Solo only as long as the business has no common-law employees other than the owner and the owner's spouse. Once a common-law employee crosses the coverage thresholds under IRC §410(a) — one year with 1,000+ hours, or (post-SECURE 2.0 §125) three consecutive years with 500+ hours — the Solo 401(k) becomes a regular 401(k) subject to ADP/ACP nondiscrimination testing, Form 5500, and coverage rules.
What is the mandatory employer contribution under a SIMPLE IRA?
SIMPLE IRAs require ONE of two employer contribution formulas each year, elected via Form 5304-SIMPLE or Form 5305-SIMPLE. Option A: a dollar-for-dollar match up to 3% of the employee's compensation (with a step-down provision allowing 1% in 2 out of 5 years). Option B: a 2% nonelective contribution to every eligible employee, up to the $360,000 comp cap = $7,200 max per employee. SECURE 2.0 §116 also allows an additional 10% nonelective contribution up to $5,000 per employee.
Which plan lets me contribute the most on a $50,000 income?
At $50,000 of Schedule C net profit or W-2 wages, the Solo 401(k) wins by roughly 3.5x. Solo 401(k) = $24,500 elective deferral + $9,294 employer = $33,794 total. SIMPLE IRA = $17,000 (or $18,100 enhanced) + $1,500 match = $18,500–$19,600 total. SEP-IRA = $9,294 total.
Can I switch from a SIMPLE IRA to a Solo 401(k) mid-year?
No — SIMPLE IRAs have a strict exclusive-plan rule under IRC §408(p)(2)(D). The switch has to be planned for the following January 1. Give employees a 60-day notice by November 2, let the SIMPLE run through December 31, then open the Solo 401(k) on January 1. SECURE 2.0 §332 created a narrow mid-year replacement provision for SIMPLE-to-Safe-Harbor-401(k) conversions but SIMPLE-to-Solo still requires the January-1 wall.
Do all three plans allow Roth contributions in 2026?
Yes. Solo 401(k) has permitted Roth elective deferrals since 2006 (IRC §402A); SECURE 2.0 §604 added Roth employer contributions in 2023. SEP-IRA — SECURE 2.0 §601 authorized Roth SEP contributions effective 2023, though custodian rollout has been slow. SIMPLE IRA — SECURE 2.0 §601 also authorized Roth SIMPLE contributions effective 2023.
What is the deadline to establish each type of plan for 2026?
SIMPLE IRA: October 1, 2026 (hard deadline, no extension). SEP-IRA: tax filing deadline including extensions (October 15, 2027 for a 2026 sole-proprietor). Solo 401(k): plan document adopted by December 31, 2026; employer contributions funded by tax filing deadline + extensions (October 15, 2027) per SECURE Act §201.
Does a SIMPLE IRA aggregate with other IRAs for the backdoor Roth pro-rata rule?
Yes. SIMPLE IRAs, SEP-IRAs, Traditional IRAs, and Rollover IRAs all aggregate under IRC §408(d)(2) for the Roth conversion pro-rata calculation. Only Solo 401(k) and other qualified plan balances under §401(a) are excluded. This is one of the strongest arguments for high-income freelancers to roll their SIMPLE or SEP into a Solo 401(k) before December 31 of the year they want to execute a clean backdoor Roth.
What happens to the SIMPLE IRA two-year withdrawal penalty?
Under IRC §72(t)(6), withdrawals or rollovers from a SIMPLE IRA within 2 years of the participant's first contribution are subject to a 25% early-distribution penalty (not the standard 10%), unless the participant is over 59½ or another §72(t) exception applies. Rollovers during the 2-year window are restricted to SIMPLE-to-SIMPLE only. After 2 years, standard IRA rollover and 10% penalty rules apply.
Methodology & sources
Every numeric claim in this article is either taken from a primary IRS or SSA source, or worked out algebraically from the statutory formula. The 2026 contribution limits are drawn from IRS Notice 2025-67 and IRS News Release IR-2025-176 (October 24, 2025). The SECURE 2.0 §117 enhanced limits are from the Congressional statutory text (Public Law 117-328, Division T). Case-study math uses the standard §401(c)(2) reduced-rate algebra and the 15.3% SE-tax formula on Schedule C net earnings × 0.9235, with the 2026 Social Security wage base of $184,500 applied to the 12.4% SS portion.
Projections of retirement balances use a 7% real annual return, consistent with the long-term S&P 500 real return of roughly 6.9% since 1928 per Aswath Damodaran's NYU Stern database, rounded up 10 bps to reflect the historical dividend-plus-inflation compensation. Actual retirement outcomes will vary with market conditions and are not guaranteed.
Primary sources
- IRS Notice 2025-67, 2026 Amounts Relating to Retirement Plans and IRAs (October 2025) — official cost-of-living-adjusted contribution limits for 2026.
- IRS News Release IR-2025-176, "401(k) limit increases to $24,500 for 2026; IRA limit increases to $7,500" (October 24, 2025) — plain-language summary of the 2026 limit changes.
- IRS, "Retirement topics — SIMPLE IRA contribution limits" — official IRS summary of SIMPLE IRA employee-deferral, catch-up, and enhanced limits under SECURE 2.0.
- IRS Publication 560, Retirement Plans for Small Business (SEP, SIMPLE, and Qualified Plans) — reference document for SEP-IRA, SIMPLE IRA, and Solo 401(k) rules including Chapter 5 Deduction Worksheet for Self-Employed.
- IRS Publication 4334, SIMPLE IRA Plans for Small Businesses — IRS practitioner guide covering eligibility, employer-contribution formulas, and deadlines.
- IRS Form 5305-SIMPLE, Savings Incentive Match Plan for Employees of Small Employers — official employer plan-document template for SIMPLE IRA adoption.
- IRS Revenue Procedure 2024-40, 2025 Inflation Adjustments (October 2024) — official federal income-tax bracket schedules referenced in case-study marginal-rate math.
- Consolidated Appropriations Act, 2023 (Public Law 117-328), Division T — SECURE 2.0 Act of 2022 — full statutory text including §109 super catch-up, §116 additional nonelective, §117 110% enhanced limit, §125 long-term-part-time coverage, §332 SIMPLE-to-Safe-Harbor conversion, §601 Roth SEP/SIMPLE, §603 mandatory Roth catch-up, and §604 Roth employer contributions.
- Aswath Damodaran, NYU Stern, Historical Returns on Stocks, Bonds and Bills — long-term S&P 500 real return data used for retirement-balance projections.
- IRS, "Retirement topics — Catch-up contributions" — official IRS summary of age 50+ catch-up and age 60–63 super catch-up rules.
- IRS Form 5500-EZ Instructions (2024) — filing thresholds ($250,000 asset trigger) and penalty structure ($250/day, max $150,000) under IRC §6652(e).
- Social Security Administration, "Contribution and Benefit Base" — official 2026 Social Security wage base of $184,500.
- IRS, "SIMPLE IRA plan frequently asked questions" — official guidance on the 60-day notice, exclusive-plan rule, and 2-year clock.
- IRS, "Independent contractor (self-employed) or employee?" — reference for the 20-factor common-law employee test that determines whether a hire triggers Solo 401(k) disqualification.
- IRS Form 5305-SEP, Simplified Employee Pension — Individual Retirement Accounts Contribution Agreement — official employer plan-document template for SEP-IRA adoption.
Last verified against IRS Notice 2025-67 and IRS News Release IR-2025-176 on July 6, 2026. Numbers may update if the IRS issues corrections, revenue procedures, or additional guidance. Consult your CPA or tax attorney before executing any of the strategies described.
Related reading on CalcLeap: SEP-IRA vs Solo 401(k) for freelancers in 2026 · Backdoor Roth IRA in 2026: the pro-rata trap and mega backdoor variant · Roth conversion ladder: the early retirement playbook · 401(k) employer match: how to never leave money on the table · Self-employed taxes: the 1099 survival guide · How much do I need to retire?