Mega Backdoor Roth Calculator
Find your real after-tax 401(k) capacity for Tax year 2026 — the $72,000 415(c) limit minus your deferrals, employer match and forfeitures — plus the age-based catch-up that applies to you, then project the Roth growth vs. a comparable taxable brokerage account.
$37,500 after-tax capacity convertible to Roth this year
Tax year 2026 415(c) annual additions limit is $72,000 (no catch-up applies — under 50). After $34,500 in deferrals, match and forfeitures, you have $37,500 of after-tax room this year.
At age 65: converting this capacity every year and growing it in-plan (100% tax-free) projects to $3,790,239, vs. $3,113,574 in a comparable taxable brokerage account — a $676,665 tax-free advantage.
Year-by-year projection
| Year | Age | Contributed to date | Roth balance | Taxable balance |
|---|---|---|---|---|
| 1 | 36 | $37,500 | $40,125 | $39,731 |
| 2 | 37 | $75,000 | $83,059 | $81,827 |
| 3 | 38 | $112,500 | $128,998 | $126,426 |
| 4 | 39 | $150,000 | $178,153 | $173,680 |
| 5 | 40 | $187,500 | $230,748 | $223,745 |
| 6 | 41 | $225,000 | $287,026 | $276,789 |
| 7 | 42 | $262,500 | $347,243 | $332,990 |
| 8 | 43 | $300,000 | $411,675 | $392,534 |
| 9 | 44 | $337,500 | $480,617 | $455,621 |
| 10 | 45 | $375,000 | $554,385 | $522,461 |
| 11 | 46 | $412,500 | $633,317 | $593,279 |
| 12 | 47 | $450,000 | $717,774 | $668,310 |
| 13 | 48 | $487,500 | $808,143 | $747,806 |
| 14 | 49 | $525,000 | $904,838 | $832,032 |
| 15 | 50 | $562,500 | $1,008,302 | $921,269 |
| 16 | 51 | $600,000 | $1,119,008 | $1,015,816 |
| 17 | 52 | $637,500 | $1,237,464 | $1,115,988 |
| 18 | 53 | $675,000 | $1,364,211 | $1,222,121 |
| 19 | 54 | $712,500 | $1,499,831 | $1,334,568 |
| 20 | 55 | $750,000 | $1,644,944 | $1,453,706 |
| 21 | 56 | $787,500 | $1,800,215 | $1,579,933 |
| 22 | 57 | $825,000 | $1,966,355 | $1,713,670 |
| 23 | 58 | $862,500 | $2,144,125 | $1,855,365 |
| 24 | 59 | $900,000 | $2,334,339 | $2,005,490 |
| 25 | 60 | $937,500 | $2,537,868 | $2,164,548 |
| 26 | 61 | $975,000 | $2,755,643 | $2,333,070 |
| 27 | 62 | $1,012,500 | $2,988,663 | $2,511,619 |
| 28 | 63 | $1,050,000 | $3,237,995 | $2,700,791 |
| 29 | 64 | $1,087,500 | $3,504,779 | $2,901,220 |
| 30 | 65 | $1,125,000 | $3,790,239 | $3,113,574 |
Tax year 2026 415(c) annual additions limit $72,000; standard catch-up (50-59 or 64+) $8,000; super catch-up (60-63) $11,250 — IRS Notice 2025-67, retrieved 2026-07-17. Requires your 401(k) plan to allow after-tax contributions AND in-plan Roth conversion (or in-service withdrawal) — not every plan does; confirm with your plan administrator. Taxable-account comparison applies a simplified annual tax drag; not a full tax calculation. Educational estimate, not tax or investment advice. How we calculate →
What a mega backdoor Roth actually is
A mega backdoor Roth lets you push far more into a Roth than the regular $7,500 Roth IRA limit — by making after-tax (not Roth, not pre-tax) contributions inside your 401(k) up to the total 415(c) annual additions limit, then converting that after-tax money to Roth via an in-plan conversion or in-service withdrawal. For Tax year 2026, that combined limit is $72,000 across your own deferrals, employer match/contributions, forfeitures, and after-tax dollars — set by IRS Notice 2025-67.
The formula behind your after-tax capacity
Capacity = 415(c) total limit − your elective deferrals (pre-tax + Roth 401(k) combined) − employer match/contributions − forfeitures allocated to you. Worked example from the IRS's own 2026 figures: a saver under 50 who maxes their $24,500 elective deferral and gets $10,000 of employer match has $72,000 − $24,500 − $10,000 = $37,500 of after-tax room left to contribute and convert. The calculator runs this exact formula on your numbers, then subtracts anything you've already contributed this year.
Catch-up contributions change the total — but only one applies
If you're 50-59 or 64+, the Tax year 2026 standard catch-up of $8,000 raises your total 415(c) limit to $80,000. If you're 60-63, SECURE 2.0's "super catch-up" of $11,250 applies instead (not on top of the standard catch-up), for a total of $83,250. Note the super catch-up is unchanged from 2025 — no COLA increase this year for that specific bracket. Under SECURE 2.0, any catch-up made by someone who earned more than $150,000 in FICA wages the prior year must go in as Roth (after-tax), which dovetails naturally with the mega backdoor strategy.
Your plan has to allow it — check before you count on it
The mega backdoor Roth only works if your 401(k) plan documents permit two specific features: (1) after-tax (non-Roth, non-pre-tax) employee contributions above the regular elective deferral limit, and (2) an in-plan Roth conversion or in-service withdrawal that lets you move that after-tax money to Roth quickly — ideally before it earns much taxable growth. Many plans, especially at smaller employers, don't offer either feature. Call your plan administrator or check your summary plan description before assuming you have this capacity.
How this differs from the classic backdoor Roth IRA
Don't confuse this with the classic "backdoor Roth IRA" — a completely separate maneuver for high earners who are phased out of contributing directly to a Roth IRA. For Tax year 2026, that phase-out is $153,000-$168,000 MAGI for single filers, and $242,000-$252,000 for married filing jointly. The classic backdoor Roth contributes to a traditional IRA (up to $7,500) and converts it — and can trigger the pro-rata rule if you hold other pre-tax IRA money. The mega backdoor Roth lives entirely inside your 401(k), operates on a much larger limit, and has no pro-rata rule (401(k) after-tax sub-accounts aren't mixed with IRA pro-rata calculations).
What this calculator doesn't model
This is an educational estimate. It assumes your plan allows after-tax contributions and in-plan conversion, that you can repeat the same contribution capacity every year until retirement (in reality, your deferrals, match and the 415(c) limit itself can all change year to year — the limit is inflation-indexed), and it doesn't account for any taxable growth that accrues on after-tax money before you convert it (ideally minimal if you convert promptly). The taxable-account comparison uses a simplified annual tax drag, not a full tax return. Talk to a qualified financial or tax advisor and your plan administrator before acting on this.
Frequently asked questions
What is the 415(c) limit for a mega backdoor Roth in 2026?
$72,000 total across your deferrals, employer match/contributions, forfeitures, and after-tax dollars — per IRS Notice 2025-67. With the standard catch-up (age 50-59 or 64+) it's $80,000; with the super catch-up (age 60-63) it's $83,250.
How much after-tax can I contribute for a mega backdoor Roth?
Capacity = $72,000 − your elective deferrals − employer match/contributions − forfeitures. For example, maxing the $24,500 elective deferral with $10,000 of employer match leaves $37,500 of after-tax room. The calculator does this math for your exact numbers.
Do the 50+ catch-up and the 60-63 super catch-up stack?
No. The super catch-up ($11,250 for ages 60-63) replaces the standard catch-up ($8,000) — you never get both in the same year. Once you turn 64, you're back to the standard catch-up amount.
Is a mega backdoor Roth the same as a backdoor Roth IRA?
No — the backdoor Roth IRA is for high earners phased out of the $7,500 Roth IRA limit; it converts a traditional IRA and can trigger the pro-rata rule. The mega backdoor Roth uses after-tax 401(k) contributions up to the much larger $72,000 415(c) limit and has no pro-rata rule.
Does my 401(k) plan automatically support this?
No — your plan must explicitly allow after-tax (non-Roth) contributions above the regular elective deferral, plus an in-plan Roth conversion or in-service withdrawal. Many plans don't offer either. Confirm with your plan administrator before relying on this strategy.
What happens if I over-contribute past the 415(c) limit?
Excess annual additions can trigger corrective action by your plan and potential tax consequences. The calculator flags when your deferrals, match, forfeitures and after-tax contributions together exceed the 415(c) limit so you can catch it before it happens.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →