Roth 401(k) vs Traditional 401(k) in 2026: The Complete Bracket-Arbitrage, Break-Even, and SECURE 2.0 §603 Guide

Almost every 401(k) plan in America now offers both buckets. Eighty-six percent of Vanguard-recordkept plans allowed Roth contributions at year-end 2024, up from about 50% a decade earlier, and eighteen percent of participants actually use the Roth option — an all-time high.^[1] But among the eighty-two percent who default into Traditional, most did so without ever running the numbers, and among the eighteen percent using Roth, at least half made the choice from a hunch rather than a calculation. The Roth-vs-Traditional decision is genuinely one of the highest-stakes personal finance calls most workers make. Over a 30-year career it can shift the after-tax retirement number by 20% or more in either direction.

The core reason is that the two buckets are governed by opposite tax mechanics. Traditional 401(k) deferrals reduce your current-year taxable wages under IRC §402(g), grow tax-deferred, and are taxed at ordinary rates in retirement. Roth 401(k) deferrals under IRC §402A do not reduce current-year wages, grow tax-free, and are withdrawn tax-free after age 59½ once the 5-year rule under §402A(d)(2) is satisfied.^[2] The mathematical answer to which wins reduces to a comparison of two marginal tax rates: yours today, versus yours in retirement. The complication is that no one knows what statutory rates will look like in 25 years, or what income the taxpayer will have then, and that uncertainty is the reason a growing share of participants split their contribution between the two buckets rather than optimize for one.

This guide walks the bracket-arbitrage math in detail, including the break-even calculation that most simplified explanations get subtly wrong. It covers the 2026 contribution mechanics under IRS Notice 2025-67, the SECURE 2.0 Act §603 mandatory Roth catch-up beginning this year for anyone whose prior-year FICA wages exceeded $150,000, the SECURE 2.0 §325 RMD elimination that quietly makes the Roth 401(k) more valuable than the Roth IRA in some ways, the SECURE 2.0 §604 employer-match Roth option and why almost no one takes it, three worked case studies at $65K / $180K / $340K income, and the six mistakes that eat 30 basis points of lifetime return.

When you want to run your own numbers, our 401(k) calculator, retirement calculator, and paycheck calculator handle the projections. The Roth conversion calculator is useful when weighing whether to convert existing Traditional balances mid-career.

The one-sentence tradeoff

The Traditional 401(k) deducts your tax bill today; the Roth 401(k) prepays it. If your marginal rate today is higher than the marginal rate you will face on the withdrawal in retirement, Traditional wins. If today's rate is lower, Roth wins. Everything else in the decision — RMDs, tax diversification, employer match treatment, the mandatory Roth catch-up rule — is a modifier on that core arbitrage, sometimes worth several percentage points of lifetime return but always secondary to the bracket call.

The 2026 contribution limits, side by side

IRS Notice 2025-67 (October 2025) published all 2026 retirement plan limits. The 401(k) elective deferral limit under IRC §402(g) rose to $24,500, up from $23,500 in 2025 — a 4.3% increase reflecting continuing above-target inflation.^[3] The single ceiling applies jointly to Traditional and Roth 401(k) contributions. You cannot contribute $24,500 to each; you can contribute $24,500 total, split any way you choose between the two buckets.

Figures verified against IRS Notice 2025-67 and IRS News Release IR-2025-176.^[3]

The mechanics differ from a Traditional deferral only at the tax-withholding line. A $24,500 Traditional 401(k) deferral reduces W-2 Box 1 taxable wages by $24,500 — Boxes 3 and 5 (Social Security and Medicare wages) are unchanged, so payroll taxes still apply.^[4] A $24,500 Roth 401(k) deferral does NOT reduce Box 1; W-2 Box 1 is still your full gross wages, and federal income tax withholding is computed on the full amount. The Roth deferral is separately reported in Box 12 with Code AA (Traditional 401(k) is Code D). Both count against the same $24,500 §402(g) ceiling, and both count against the same §415(c) $72,000 total-DC-plan ceiling that also caps employer match and after-tax contributions.

The bracket-arbitrage math, in detail

The mathematical core of the Roth-vs-Traditional decision is a comparison of two after-tax final balances under the same nominal deferral. Assume you have $10,000 of pre-tax income that you want to save. Your current marginal tax rate is t_now. Your expected marginal tax rate in retirement is t_later. Your investment horizon is n years and your annual real return is r.

Traditional 401(k) path. The full $10,000 goes into the 401(k) untaxed. It grows to $10,000 × (1 + r)^n. At withdrawal, you pay tax at t_later, leaving you with:

Roth 401(k) path. You pay tax at t_now up front, so only $10,000 × (1 − t_now) actually gets deferred. That amount grows to $10,000 × (1 − t_now) × (1 + r)^n, and no tax is owed at withdrawal:

The ratio. Divide the two:

The result is invariant to r, n, and the dollar amount. All that matters is the two marginal tax rates. If t_now = t_later, the ratio is exactly 1.0 — you end up with identical after-tax retirement dollars. If t_now is lower than t_later, the ratio exceeds 1.0 and Roth wins. If t_now is higher, the ratio is under 1.0 and Traditional wins.

The 2026 tax brackets you're actually comparing

The One Big Beautiful Bill Act (OBBBA) made the TCJA ordinary income structure permanent and added an inflation catch-up adjustment for the 10% and 12% brackets specifically. IRS News Release IR-2025-176 published the resulting 2026 brackets in October 2025.^[5]

Marginal rate is what matters for the Roth-vs-Traditional decision, not effective rate. If you file jointly and your last dollar of 2026 income lands in the 24% MFJ bracket ($211,400–$403,550), the Traditional 401(k) deferral saves you 24 cents on the dollar this year regardless of what rate your first dollar of income was taxed at. Roth costs you that same 24 cents in current-year tax not-saved. In retirement, when you pull that same money out, the marginal rate on the withdrawal depends entirely on what OTHER retirement income you have that year — Social Security, pension, Traditional IRA/401(k) RMDs, dividends, rental income.

The retirement-marginal-rate estimation problem

The dominant driver of retirement marginal rate is total retirement income (all Social Security + pension + RMD + brokerage + wages), which most workers cannot forecast to within 20 percentage points. Rules of thumb that help calibrate:

• Standard-of-living rule. If you plan to spend in retirement roughly the same real amount you spend today, your retirement AGI will be about the same as your working AGI — so bracket now ≈ bracket later. Traditional and Roth tie under this assumption.
• Retirement compression rule. Most workers spend less in retirement than during peak earning years (kids launched, mortgage paid, no commute, no work wardrobe). The BLS Consumer Expenditure Survey shows household spending peaks in the 45-54 age band and falls roughly 25% by 65-74.^[6] Lower spending → lower withdrawal → lower marginal rate → Traditional wins by a modest margin.
• Legacy and RMD compression. Retirees with large Traditional balances get pushed into higher brackets by mandatory RMDs starting at 73, especially when combined with Social Security taxation and Medicare IRMAA cliffs. Roth 401(k)s never trigger this. High Traditional balances build "tax time bombs" that only Roth or in-plan Roth conversion can defuse.
• Statutory rate risk. Current U.S. marginal rates are historically low. The top rate hit 91% in the 1950s, 70% through most of the 1970s, and 50% under Reagan.^[7] Anyone who believes federal deficits or debt-to-GDP will force higher statutory rates in 25 years should adjust Roth-preference upward.

Break-even scenarios in dollar terms

Numeric intuition helps. Below is the outcome for a 30-year-old worker who defers $20,000 pretax-equivalent per year for 35 years at 7% real return, then withdraws it as retirement income at 65, under four bracket scenarios.

The break-even is exact — Scenario 2 shows that when t_now = t_later, the outcomes are identical to the dollar. This is the mathematical result of the ratio formula above, and it is the key reason a plurality of professional advisors recommend defaulting to a 50/50 split for anyone in the 22% or 24% brackets today: the downside of being wrong is small and symmetric.

Tax diversification — why splitting often beats optimizing

Even if you've done the bracket calculation and Traditional appears to win, splitting the deferral between Traditional and Roth is often the correct move under conditions of uncertainty. The argument is not mathematical optimization — it is regret minimization.

A 30-year-old worker deciding today about her 401(k) contributions has to forecast: (a) her own income trajectory over the next 35 years, (b) her spending trajectory in retirement 35 years from now, (c) statutory federal tax rates 35 years from now, and (d) statutory state tax rates 35 years from now if she moves. The joint distribution of those four unknowns has enormous variance. A 100% Traditional bet or a 100% Roth bet is a concentrated position on one specific realization of that joint distribution.

Splitting 50/50 caps your regret in the worst case. If future rates turn out much higher than today's, your Roth half sheltered you. If future rates turn out much lower, your Traditional half captured the current-year deduction at the higher rate. Under a symmetric two-outcome uncertainty distribution, the 50/50 split minimizes maximum regret at the cost of a small expected-value give-up. Under the Vanguard Center for Investor Research's participant behavioral data, when workers are offered simple UI to split a 401(k) contribution between Traditional and Roth, roughly 25-30% of participants elect a split rather than 100% of either bucket.^[1]

SECURE 2.0 §603 — the mandatory Roth catch-up for high earners

The single most important 2026-specific rule is IRC §414(v)(7), added by SECURE 2.0 Act §603, which for the first time requires that certain catch-up contributions be made on a Roth basis rather than pre-tax. This provision was originally scheduled to apply from 2024 but was delayed by IRS Notice 2023-62 to 2026 to give employers time to update payroll systems.^[8] As of January 1, 2026, it is in force.

The trigger test

The mandatory-Roth-catch-up rule applies to any employee whose FICA wages from the SAME EMPLOYER in the prior calendar year exceeded $150,000 (indexed for inflation; the 2026 threshold is $150,000). All three elements matter:

• FICA wages, not gross wages. Wages subject to Social Security tax under IRC §3121(a). Pre-tax 401(k) elective deferrals do NOT reduce FICA wages, so the threshold is measured on pre-deferral compensation. Cafeteria plan pre-tax medical premiums DO reduce FICA wages.
• Prior year, not current year. The test uses the prior calendar year's FICA wages to determine whether the current year's catch-up must be Roth. Your 2026 catch-up election depends on your 2025 FICA wages, not your 2026 income.
• Same employer, not aggregated across employers. A worker who earned $100,000 at Employer A in 2025 and $100,000 at Employer B in 2025 is BELOW the $150K threshold at each employer separately and retains full choice on catch-up in 2026 at both.

What happens if you're subject to §603 but your plan doesn't offer Roth

You cannot make catch-up contributions at all. IRS Notice 2023-62 confirmed the position that a plan not offering a Roth feature cannot accept catch-up contributions from §603-subject employees. This has driven roughly 60% of plans that did not previously offer Roth to add the feature by year-end 2025 in preparation for 2026 enforcement.^[10] If your employer's plan still does not offer Roth as of January 1, 2026 and you are §603-subject, your maximum 2026 deferral is capped at the base $24,500 with no catch-up — a real dollar cost to the employee.

The practical rewrite for §603-subject employees

An employee subject to §603 who wants to maximize their 401(k) shelter now runs two elections:

1. Base $24,500 elective deferral. The §603 rule does NOT touch this; it applies only to catch-ups. The base $24,500 can still go 100% Traditional if the employee prefers.
2. Catch-up ($8,000 age 50+, or $11,250 age 60-63). Must be 100% Roth if §603 applies.

A 55-year-old executive earning $200,000 in FICA wages at Employer A in 2025 who wants to max her 2026 401(k) will contribute $24,500 Traditional + $8,000 Roth catch-up = $32,500 total. The catch-up portion increases her 2026 W-2 Box 1 by $8,000 and creates an incremental federal tax cost at her marginal 32% rate of about $2,560. Over a full 15-year working horizon at 7% real return, that $8,000 annual Roth catch-up compounds to about $201,000 tax-free — a materially better long-run outcome than the $8,000 Traditional catch-up would have produced under the pre-§603 regime, so the forced Roth allocation is not necessarily a loss even for a high earner.

Employer match — always pre-tax by default, and why

Historically, all employer matching and nonelective contributions to a 401(k) plan had to be made on a pre-tax basis, regardless of whether the employee's elective deferral was Traditional or Roth. SECURE 2.0 Act §604 (effective 2023 onward) changed this: employers CAN offer a Roth match, if the plan document permits it AND the employee elects it. As of 2026, industry survey data suggests roughly 25% of plans have added the Roth-match option, up from about 15% at the start of 2024, but adoption among employees remains under 10% even where offered.^[1]

The reason adoption is low is straightforward. When an employer contributes $6,000 in matching dollars on a Roth basis, that $6,000 is added to the employee's W-2 Box 1 as taxable wages for the current year. At a 24% marginal federal rate, the employee owes about $1,440 in additional federal income tax on the match — cash they may not have on hand. Most workers already stretched by living costs prefer the pre-tax default and let the match grow tax-deferred, deferring the tax bill to retirement.

What the match feels like on the pay stub

A worker earning $100,000 who defers $10,000 to Roth 401(k) with a 5% employer match ($5,000) sees the following on her 2026 W-2 assuming pre-tax match:

• Box 1 (taxable wages, federal income tax): $100,000 (no reduction, Roth deferral doesn't reduce)
• Box 3 (Social Security wages): $100,000 (Traditional or Roth, both count for FICA)
• Box 5 (Medicare wages): $100,000
• Box 12, Code AA: $10,000 (Roth 401(k) elective deferral)
• Employer match of $5,000: not on W-2 at all — it goes to the pre-tax 401(k) subaccount

Compare to the same worker deferring $10,000 to Traditional 401(k):

• Box 1: $90,000 (deferral reduces)
• Box 3: $100,000 (unchanged)
• Box 5: $100,000
• Box 12, Code D: $10,000 (Traditional 401(k) elective deferral)
• Employer match of $5,000: not on W-2 (pre-tax match, goes to pre-tax bucket)

The Roth path shows $100K taxable wages and pays federal income tax on the full amount now. The Traditional path shows $90K taxable and pays $10K less in federal income tax now, deferred to retirement. Both paths shelter $15,000 total inside retirement accounts.

RMDs — the quiet asymmetry that's growing

Required minimum distributions are the single most underappreciated Roth-vs-Traditional differentiator. Pre-2024, all 401(k) balances — Traditional AND Roth — were subject to RMDs beginning at age 73 (rising to 75 for those born in 1960 or later under SECURE 2.0 §107). The Roth 401(k) RMD requirement was widely viewed as an arbitrary quirk, because Roth IRA balances have never been subject to lifetime RMDs.

SECURE 2.0 Act §325 eliminated the Roth 401(k) lifetime RMD requirement effective 2024. As of 2026, Roth 401(k) balances behave like Roth IRA balances during the account holder's lifetime — no forced distributions, ever. Traditional 401(k) balances continue to require RMDs starting at 73/75 depending on birth year.^[11]

Why RMD elimination matters more than the tax rate call

A Traditional 401(k) worth $2 million at age 73 has a mandatory RMD in year one of approximately $75,472 ($2M / 26.5, the IRS Uniform Lifetime Table divisor).^[12] That $75,472 lands on top of Social Security, pension, dividend income, and whatever else the retiree already has. For a couple with $50,000 in Social Security benefits and $30,000 in pension income, the first $75,472 of Traditional RMD pushes them from about 12% marginal to 24% marginal and triggers the IRMAA Medicare Part B surcharge (an income-related monthly adjustment of $70-$400 per person per month depending on income tier). The cumulative effect is a much higher effective retirement marginal rate than the same couple would face if that $2 million had been Roth.

The Roth 401(k) retiree faces NONE of this. Withdrawals are optional. Withdrawals are tax-free. Withdrawals do not count toward provisional income for Social Security taxation. Withdrawals do not count toward IRMAA. The Roth 401(k) becomes the retiree's "tax-free reservoir" for lumpy expenses (a new car, a home repair, medical events) without triggering bracket-jump effects on the rest of the retirement income.

Interaction with backdoor Roth and mega backdoor Roth

Workplace Roth 401(k) contributions and backdoor/mega backdoor Roth strategies are complementary, not substitutes. Understanding how they layer prevents both under-contribution (leaving Roth capacity unused) and over-contribution (running into §402(g) or §415(c) caps by accident).

Roth 401(k) does not affect backdoor Roth IRA eligibility

Backdoor Roth IRA is a maneuver where a high earner contributes nondeductibly to a Traditional IRA under IRC §408(a) and then immediately converts to Roth IRA. It works cleanly only when the taxpayer's total pre-tax IRA balance (Traditional, SEP, SIMPLE) on December 31 is essentially zero, due to the pro-rata rule under IRC §408(d)(2).^[13] A Roth 401(k) balance is NOT an IRA balance and does NOT enter the pro-rata denominator. A worker can contribute $24,500 to a Roth 401(k) AND do a $7,500 backdoor Roth IRA in the same year, entirely independently. See our complete guide to the backdoor Roth IRA for the mechanics.

Mega backdoor Roth stacks on top of Roth 401(k)

The mega backdoor Roth uses after-tax (non-Roth) 401(k) contributions above the §402(g) $24,500 elective-deferral ceiling but within the §415(c) $72,000 total-DC ceiling, then immediately converts them to Roth via in-plan Roth rollover under IRC §402A(c)(4). A worker maxing $24,500 in Roth 401(k) elective deferrals PLUS receiving a $6,000 employer match still has room for up to $41,500 in after-tax mega backdoor contributions if the plan permits.^[14] Total 2026 Roth capacity: $24,500 (elective deferral) + $41,500 (mega backdoor) + $7,500 (backdoor Roth IRA) = $73,500 per person before employer match.

Roth 401(k) versus Roth IRA — different accounts, different rules

The Roth 401(k) is a bigger shelter than the Roth IRA on almost every dimension. It has no income phaseout (a high earner locked out of the Roth IRA can freely contribute to Roth 401(k)). Its contribution limit is $24,500 vs $7,500. It permits participant loans up to $50,000 or 50% of vested balance. It has been RMD-free since 2024. The one dimension where the Roth IRA wins: investment choice. Roth 401(k)s are limited to the plan's investment menu, which typically has 15-25 mutual funds and target-date options; Roth IRAs at Fidelity, Schwab, Vanguard, or Charles Schwab can hold any exchange-traded security. For most workers this doesn't matter — cheap index funds are available in both. See our companion piece on Roth IRA vs Traditional IRA.

Three case studies with full numbers

Case 1: Priya, 28, single, $65,000 salary — Roth-dominant, 100% Roth

Priya is a first-year associate at a mid-size consulting firm. Single filer, $65,000 gross salary in 2026, standard deduction of $16,150 (2026 standard deduction for singles per Rev. Proc. 2025-32). Her taxable income is $48,850, landing entirely in the 10% and 12% brackets.^[5] Marginal federal rate: 12%.

• Current federal marginal rate: 12%
• Employer match: 4% dollar-for-dollar up to $2,600
• State: no income tax (Texas)
• Expected retirement bracket: 22% (assumes she reaches senior consultant tier over her career and retires modestly in the 22% MFJ bracket)

Priya's Roth-vs-Traditional bracket call is unambiguous: 12% now vs 22% later. Ratio (1 − 0.12) / (1 − 0.22) = 1.128, or a 12.8% after-tax retirement balance advantage for Roth. She elects 100% Roth on her $6,500 annual deferral (10% of salary) and 100% Roth on the future age-50 catch-ups when she becomes eligible. Employer match of $2,600 goes pre-tax by default (employer offers Roth match but she declines because the current-year tax cost is real cash flow she needs).

Projection: $6,500/yr Roth for 37 years at 7% real return = $1,113,000 tax-free at 65. Plus $2,600/yr pre-tax match = $445,000 pre-tax at 65, taxable at retirement rate. Total after-tax retirement balance at assumed 22% retirement bracket: $1,113,000 + ($445,000 × 0.78) = $1,460,000. If she had gone 100% Traditional instead, the after-tax result would have been about $1,332,000, a $128,000 give-up — 8.8% of her total retirement portfolio.

Case 2: David, 42, MFJ $180,000 combined salary — 50/50 split, tax diversification

David and his spouse both work, joint 2026 income $180,000, both under 50. Standard deduction $32,300 (2026 MFJ) → taxable $147,700, landing entirely in the 22% MFJ bracket. Marginal federal rate: 22%. State: California, marginal 9.3%. Combined marginal rate: about 30%.^[5]

• Current combined federal + state marginal rate: 30% (22% federal + ~8% CA after federal deduction interaction)
• Expected retirement state: Nevada (both plan to move at 65 to no-income-tax state)
• Expected retirement federal bracket: 22% (target retirement spending $120K/yr, roughly matches current after-tax spending)
• Combined retirement marginal rate: 22% (state 0% after move)
• Employer match: 5% up to comp cap

David's bracket call: current 30% vs retirement 22%. Ratio (1 − 0.30) / (1 − 0.22) = 0.897 — Traditional wins by 10% on the pure math. But the state-move assumption is a real uncertainty (job change, family reasons, plans change), and the retirement bracket assumption is a 20-year forecast. He elects a 60% Traditional / 40% Roth split on his $24,500 deferral. This captures most of the current-year deduction benefit ($4,000 federal + state tax saved) while building a $9,800/year Roth position that hedges the possibility of retiring in California after all.

Projection over 23 years (age 42 to 65) at 7% real: Traditional balance $14,700/yr × 47.0 (annuity factor) = $691,000 pre-tax → $538,000 after-tax at 22% federal (no CA if move happens). Roth balance $9,800/yr × 47.0 = $460,600 tax-free. Total: $998,600 after-tax. Under the pure Traditional path he would have had about $1,053,000 after-tax at retirement — a $54,000 give-up in the base case in exchange for a much better outcome if state-move doesn't happen or federal rates rise.

Case 3: Elena, 58, single, $340,000 salary — subject to §603, 100% Traditional base + Roth catch-up

Elena is a senior VP at a Fortune 500 firm, single, W-2 $340,000, 2025 FICA wages were $340,000. She turns 59 in 2026, so she qualifies for the standard age-50 catch-up but not the age-60-63 super catch-up. She is subject to SECURE 2.0 §603 because her 2025 FICA wages exceeded the $145K (2025 threshold, or $150K 2026 indexed).^[8]

• Current federal marginal rate: 32% (single, taxable income about $310K)
• State: New York, marginal 6.85%. Combined: about 37%
• Expected retirement bracket: 22% (retiring at 63 with pension replacing ~50% of income)
• Employer match: 8% dollar-for-dollar up to comp cap ($360,000)

Elena's bracket call is a Traditional slam-dunk on the base deferral: 37% now vs 22% later. Ratio 0.63 / 0.78 = 0.808 — Traditional wins by 19% on the pure math for the base deferral. But §603 forces her $8,000 catch-up to Roth regardless of preference.

Her 2026 election: $24,500 base deferral 100% Traditional + $8,000 catch-up 100% Roth (mandatory) = $32,500 total. Plus $8,000 mandatory-Roth catch-up increases her 2026 W-2 Box 1 by $8,000 vs a fully-pre-tax equivalent, costing her $8,000 × 37% = $2,960 in current-year federal + state tax. On a 5-year horizon (she retires at 63) the $8,000/yr Roth catch-up at 6% real return grows to $47,750 tax-free, versus a $8,000/yr Traditional catch-up growing to $47,750 pre-tax → $37,245 after retirement tax. The Roth path wins by $10,505 over her final 5 working years even though the current-year cost is $2,960/year. Cumulative current-year cost: $14,800. Cumulative retirement value advantage: $10,505. Net: minor Roth disadvantage in Elena's case, but the deficit is small enough to accept given the §603 mandate.

Six mistakes that cost 30+ basis points

1. Choosing Roth in a high bracket because "tax rates will go up"

The bracket call decides most of the arithmetic. If your CURRENT marginal rate is materially higher than what you'll face in retirement, Roth loses money in the base case even if statutory rates rise. Only when rates rise enough to close a 10-point gap does Roth catch up. Fix: run the actual bracket comparison first, then adjust for statutory risk. Statutory risk is a modifier, not the primary driver.

2. Contributing 100% Roth as a peak-earner in the 32%+ bracket without a diversification allocation

Even under generous assumptions about future rates, a 32%-bracket worker contributing 100% Roth is giving up 10+ points of expected value versus 100% Traditional. If tax diversification is the goal, cap the Roth allocation at 20-30% of the deferral. Fix: 70/30 Traditional/Roth for peak earners preserves most of the current-year deduction while building tax-free capacity.

3. Confusing Roth 401(k) with after-tax non-Roth 401(k)

Roth 401(k) and after-tax non-Roth 401(k) are different buckets. Roth 401(k) contributions are already Roth — no conversion needed, no tax on growth. After-tax non-Roth contributions grow TAXABLE unless converted to Roth via in-plan Roth rollover (mega backdoor). Contributing to the wrong bucket by mistake creates permanent tax exposure on the growth. Fix: verify with your plan portal or HR that your incremental contribution is going to the correct subaccount. The Roth 401(k) is the "designated Roth" bucket (Box 12 Code AA on W-2); the after-tax bucket is separate.

4. Ignoring §603 and finding out via a plan-error notice in Q2 2026

A §603-subject employee who elected pre-tax catch-up in November 2025 for 2026 and had payroll process the catch-up as pre-tax has an operational error. The plan sponsor must recharacterize the catch-up as Roth (adding it to W-2 Box 1) or refund it entirely. Fix: verify with HR that your 2026 catch-up election is Roth if you're §603-subject. Employers are required to give employees a §603 warning in their 2026 open enrollment materials, but many buried it in a footnote.

5. Rolling a Roth 401(k) to a Roth IRA at retirement without checking the 5-year clock

The 5-year clock on a Roth 401(k) does NOT transfer to a Roth IRA on rollover. The Roth IRA's own separate 5-year clock (based on the FIRST Roth IRA contribution to any Roth IRA in your lifetime) governs. If you never had a Roth IRA before rolling in your $500,000 Roth 401(k), you start a fresh 5-year clock at 65. Withdrawing earnings before 70 triggers ordinary income tax on the earnings portion. Fix: open a $1,000 Roth IRA at any custodian at least 5 years before you plan to retire, purely to start the clock — an easy 5-minute defensive move.

6. Skipping the employer match to prioritize Roth

The employer match is a 100% (or 50%) instant return on your deferral. Nothing beats that yield. If your plan only offers a pre-tax match and you REALLY want Roth exposure, do the match up to the maximum first (in Traditional) THEN allocate the remainder of the $24,500 to Roth. Skipping the match to keep all your contributions Roth costs several times more than the Roth-vs-Traditional bracket call. Fix: match first, bucket allocation second.

Your 8-item action checklist for 2026

1. Compute your 2026 marginal federal + state rate today. Use your 2025 tax return as a baseline. Add state marginal rate (net of federal deductibility). This is t_now.
2. Estimate your marginal retirement rate. Assume similar real spending, apply the 2026 brackets (they're permanent under OBBBA), account for Social Security taxation and RMD stacking. This is t_later. If the two are within 3 points of each other, split 50/50.
3. Verify your §603 status. If your 2025 FICA wages from your current employer exceeded $145,000 (2025 threshold indexed to $150K for 2026 test), your 2026 catch-up MUST be Roth. Election deadline is typically your plan's fall open-enrollment window; if you missed it, contact HR immediately.
4. Max the employer match first. Whatever your Roth-vs-Traditional split, contribute enough Traditional to capture the full match. Match dollars are a guaranteed 50-100% return that dominates the bracket call.
5. Set your bucket allocation with a written thesis. Write down (on paper or in a spreadsheet): "I'm allocating X% Traditional / Y% Roth in 2026 because [my rate today is / my expected retirement rate is / my tax-diversification target is]." Revisit annually.
6. Check whether your plan offers mega backdoor Roth. Ask HR: "Does the plan permit after-tax non-Roth employee contributions above the $24,500 elective deferral limit? Does it allow in-plan Roth conversion or in-service distribution of the after-tax bucket?" If both yes, you have up to $47,500 additional Roth capacity.
7. Start a $1,000 Roth IRA if you don't have one. Purely defensive — starts your Roth IRA 5-year clock, which is separate from the Roth 401(k) 5-year clock and matters when you eventually roll the workplace Roth to an IRA at retirement.
8. Automate the deferral election and revisit every October. Bracket allocations should be reviewed once a year, at open enrollment, against your projected income for the following year. Life events (marriage, promotion, layoff, job change) trigger an off-cycle review.

Frequently asked questions