How much can you put in a mega backdoor Roth?
Your after-tax 401(k) space is the $72,000 total limit minus your deferral and employer match — up to $47,500 extra into Roth for 2026. Enter your numbers to see your exact room.
For 2026, the mega backdoor Roth lets you contribute after-tax 401(k) dollars up to the $72,000 §415(c) annual-additions limit, then convert them to Roth. Your room = $72,000 − your elective deferral (max $24,500) − employer contributions. With a full deferral and no employer match that's $47,500 of extra Roth space; a typical $10,000 match leaves about $37,500. Your plan must allow after-tax contributions plus in-plan conversion or in-service withdrawals.
Your 401(k) plan inputs (2026)
2026 limit: $24,500. This is what comes out of your paycheck into the 401(k).
Everything your employer puts in. Check a recent pay stub or last year's W-2 / plan statement.
Your mega backdoor Roth space
How your $72,000 annual-additions limit fills up
blue = your deferral · sky = employer · green = after-tax roomHow the mega backdoor Roth works
A standard 401(k) caps what you can defer from your paycheck at $24,500 in 2026. But the IRS sets a much higher ceiling on total annual additions to your account — the section 415(c) limit of $72,000, which counts your deferral, every dollar your employer contributes, and any after-tax contributions. The gap between those two numbers is the opportunity.
The mega backdoor Roth fills that gap with after-tax money and immediately converts it to Roth. Step one: contribute after-tax (a third bucket, distinct from pre-tax and Roth deferrals) up to the $72,000 ceiling. Step two: convert those after-tax dollars to a Roth IRA (via in-service withdrawal) or a Roth 401(k) (via in-plan conversion) before they accrue taxable earnings. Once in Roth, all future growth is tax-free forever.
The math the calculator runs is simple but easy to get wrong by hand: after-tax space = $72,000 − your elective deferral − employer contributions. The more your employer contributes, the less after-tax room you have — a generous match is great, but it eats into your mega backdoor capacity dollar for dollar.
Two things your plan must allow
- After-tax contributions beyond the $24,500 deferral limit. This is a separate election from your pre-tax and Roth deferrals — many plans simply don't offer it.
- In-plan Roth conversion or in-service withdrawal, so you can move the after-tax money into Roth promptly. The best plans auto-convert each contribution; otherwise convert manually and often, because only the principal converts tax-free — accrued earnings are taxable.
Check your Summary Plan Description or ask your plan administrator. If either feature is missing, the mega backdoor isn't available to you this year — max your regular $24,500 deferral and a $7,500 backdoor Roth IRA instead.
Frequently asked questions
What is a mega backdoor Roth?
The mega backdoor Roth is a strategy that lets high earners move far more into Roth accounts than the normal $24,500 2026 401(k) limit. It works in two steps: (1) make after-tax (non-Roth, non-pre-tax) contributions to your 401(k) up to the overall §415(c) annual-additions limit of $72,000, then (2) convert those after-tax dollars to a Roth IRA or Roth 401(k) — either via an in-plan Roth conversion or an in-service withdrawal. The result can be up to $47,500 of extra Roth contributions per year on top of your regular deferral.
How much can I contribute to a mega backdoor Roth in 2026?
Your after-tax space equals the $72,000 §415(c) total limit minus your own elective deferral minus all employer contributions. If you defer the full $24,500 and get no employer match, you have $47,500 of after-tax room. A typical worker with a $10,000 employer match would have about $37,500. Catch-up contributions (age 50+) sit on top of the §415(c) limit, raising the ceiling further.
Does my 401(k) plan allow a mega backdoor Roth?
Not all plans do — it requires two specific features: (1) the plan must permit after-tax contributions beyond the standard deferral limit, and (2) it must allow either in-plan Roth conversions or in-service withdrawals so you can move that after-tax money into Roth before it accumulates taxable earnings. Check your Summary Plan Description or ask HR/your plan administrator. Roughly 40–50% of large-employer plans support it; many small-employer plans do not.
Is the mega backdoor Roth the same as a regular backdoor Roth?
No. The regular backdoor Roth is for the $7,500 IRA limit: you make a non-deductible Traditional IRA contribution and convert it to Roth, useful when your income is too high to contribute to a Roth IRA directly. The mega backdoor Roth happens inside your 401(k) and moves far more money — up to $47,500 versus $7,500. Many high earners do both in the same year.
Why bother — what's the tax benefit?
Without the mega backdoor, after-tax 401(k) money still grows tax-deferred, but the growth is taxed as ordinary income on withdrawal. By converting it to Roth promptly, all future growth becomes completely tax-free, and qualified Roth withdrawals are tax-free in retirement. Over 20–30 years, the tax-free growth on $40K+/year of contributions can reach seven figures — money you'd otherwise share with the IRS in a taxable brokerage account.
When should I convert the after-tax money to Roth?
As soon as possible after each after-tax contribution. After-tax dollars that sit in the 401(k) accrue earnings, and those earnings ARE taxable when you convert (only the after-tax principal converts tax-free). Plans that support automatic in-plan Roth conversion (sometimes called 'auto in-plan Roth rollover') eliminate this problem by converting each contribution immediately. If your plan requires manual conversions, do them quarterly at minimum.
Does this calculator include state taxes or the pro-rata rule?
No. After-tax 401(k) contributions are made with money you've already paid income tax on, so there's no additional federal or state income tax on the contribution or the conversion of principal — only on any earnings that accrued before conversion. The pro-rata rule that complicates IRA backdoor conversions does not apply to in-plan 401(k) after-tax conversions. This tool models contribution capacity, not your full tax return — consult a CPA for your specific plan.