Mega Backdoor Roth Calculator (2026)

What this calculator models

The mega backdoor Roth is a workplace-plan strategy that lets a high-income participant push tens of thousands of after-tax dollars into a Roth account each year — far more than the $7,500 Roth IRA base limit or the $24,500 Roth 401(k) elective-deferral cap. The mechanic exploits IRC §415(c), which caps total annual additions to a defined-contribution plan at $72,000 for 2026 per IRS Notice 2025-67, while allowing the after-tax source to fill the space between your elective deferral plus employer contribution and that ceiling. A subsequent in-plan Roth rollover under IRC §402A(c)(4)(E) or an in-service withdrawal converts the after-tax dollars to Roth without a distribution event.

The calculator does five things: (1) checks your plan supports both required features via the pre-check checkboxes; (2) computes your after-tax capacity as $72,000 minus base elective deferral minus employer contribution; (3) applies the IRC §414(v)(3)(A) catch-up-outside-§415(c) exception so age-50 and super catch-up contributions do not eat into your after-tax room; (4) applies the SECURE 2.0 §603 mandatory Roth catch-up gate for prior-year wages above $150,000; (5) quantifies the same-day-vs-quarterly conversion earnings drag using the standard formula (after-tax) × return ÷ (2 × sweeps-per-year).

The 2026 numbers you need to know

• IRC §402(g) elective deferral limit: $24,500 base for 2026 (Notice 2025-67).
• IRC §414(v)(2)(B)(i) age-50 catch-up: $8,000 additional. Available at age 50 through 59 and 64+.
• IRC §414(v)(2)(E)(i) super catch-up: $11,250 additional. Available at ages 60–63 only.
• IRC §415(c) annual additions ceiling: $72,000 for 2026. Includes elective deferral + employer contribution + after-tax employee contribution. Excludes catch-up (§414(v)(3)(A)).
• IRC §401(a)(17) compensation cap: $360,000. Employer percentage-of-pay formulas apply only to the first $360,000 of your compensation.
• IRC §414(v)(7) mandatory Roth catch-up threshold (§603): $150,000 prior-year FICA wages from the plan sponsor. Over the threshold, all catch-up must be Roth.

How the after-tax capacity math works

Your after-tax capacity under §415(c) is straightforward: capacity = $72,000 − base_elective − employer_contribution. A 42-year-old at a plan that gives them $9,000 in employer match plus their own $24,500 elective deferral has $72,000 − $24,500 − $9,000 = $38,500 of after-tax room. A 42-year-old with no employer match doing the full $24,500 elective has $72,000 − $24,500 − $0 = $47,500 of after-tax room. If your employer contribution is $0 and your elective deferral is $0, the entire $72,000 could theoretically be after-tax — though most plans do not permit that pattern operationally.

The catch-up (age 50+) sits outside the §415(c) cap under §414(v)(3)(A). So a 62-year-old at a plan with no employer match doing the full base elective as Roth can accumulate: $24,500 Roth deferral + $11,250 mandatory Roth super catch-up + $47,500 after-tax mega backdoor = $83,250 of Roth in one calendar year. This is the top-of-the-cluster Roth capacity number for 2026 — worth quantifying because it does not appear on most competing calculators.

The SECURE 2.0 §603 mandatory Roth catch-up gate

Effective 2026 after the transition relief in IRS Notice 2023-62 expired at the end of 2025, IRC §414(v)(7) requires that if you are age 50 or older AND had FICA wages exceeding $150,000 from the plan sponsor in the prior calendar year, your catch-up contribution must be designated Roth. Pre-tax catch-up is unavailable to you. If your plan does not offer designated Roth catch-up, the §603 gate makes your catch-up contribution altogether unavailable — a plan-design failure the calculator flags. The prior-wages threshold is per-employer, so a mid-year job change can reset your §603 status.

The ACP test problem at small employers

The mega backdoor Roth's biggest structural risk is not at large FAANG-style employers with tens of thousands of participants — it's at small employers whose after-tax subaccount is disproportionately used by highly compensated employees. IRC §401(m)(2) applies a 1.25× ACP nondiscrimination test: the average contribution percentage for HCEs may not exceed 1.25 × the average for NHCEs. In a plan where NHCEs contribute virtually nothing to the after-tax source, HCEs cannot either. Safe Harbor 401(k) status does NOT exempt the after-tax bucket from ACP — a common misconception. Ask your TPA or plan sponsor for the ACP test results before you plan on the strategy.

Sweep frequency and earnings drag

After-tax dollars in the plan earn taxable growth. When you finally convert to Roth, IRC §72(e) makes those accumulated earnings ordinary income. The mechanical solution is a same-day sweep — many large plan sponsors offer automatic in-plan Roth rollover the day after-tax dollars hit the subaccount, so earnings never accumulate. Where same-day is not available, monthly is materially better than quarterly, which is materially better than annual. The calculator quantifies the drag directly using: annual earnings drag = after_tax × return ÷ (2 × sweeps_per_year). For $47,500 at 8% return: same-day ≈ $0, monthly = $158, quarterly = $475, annual = $1,900. Multiply by (federal marginal + state) to get the avoidable tax.

Related tools and reading

• Mega backdoor Roth in 2026: the §415(c) $72,000 ceiling, the two plan features, ACP test, and the $83,250 max-Roth year — the full guide this calculator serves.
• Backdoor Roth IRA Calculator — the $7,500 IRA-based cousin strategy for filers whose plan does not offer the mega backdoor.
• Backdoor Roth IRA in 2026: the pro-rata trap — the standard-backdoor strategy article, worth reading if you have any rollover IRA balance.
• How to file Form 8606 in 2026 — reporting paperwork if you also run the standard backdoor alongside.
• Roth 401(k) vs Traditional 401(k) in 2026 — decision on the elective portion.
• Safe Harbor 401(k) in 2026 — why Safe Harbor does NOT exempt the after-tax bucket from ACP.
• 401(k) Calculator — full elective + employer + catch-up projection with §603 model.
• Roth IRA Calculator — direct Roth projection when MAGI is under phaseout.
• Roth IRA Conversion Calculator — year-by-year projection of a larger Traditional-to-Roth conversion.
• SEP IRA Calculator and SIMPLE IRA Calculator — self-employed / small-employer alternatives.
• Retirement Calculator — combined-account projection.