Mega Backdoor Roth Calculator

The Biggest Roth Pipe in the Tax Code

Three buckets, one ceiling

401(k) plans hold three contribution types: your deferrals (2025 limit $23,500), employer money, and — where the plan allows — after-tax contributions, all sharing one §415(c) ceiling of $70,000 ($77,500 with age-50 catch-up). Most savers max the first bucket and stop, unaware the third exists. After-tax contributions alone are mediocre (gains grow tax-deferred but come out as ordinary income); converting them to Roth is what turns mediocre into exceptional — hence 'mega backdoor.'

What your plan must offer

Two plan features are non-negotiable: after-tax (non-Roth) contributions, and either in-plan Roth conversions or in-service distributions to an external Roth IRA. Big-tech and large-employer plans commonly have both; small-business plans often don't, partly because after-tax contributions must pass ACP nondiscrimination testing — highly-paid employees can have contributions returned if rank-and-file participation lags. A 60-second HR question ('do we allow after-tax contributions with in-plan Roth conversion?') settles eligibility.

Why the pro-rata rule doesn't bite here

Unlike the regular backdoor Roth, the mega version lives inside the 401(k), where IRA pro-rata rules don't reach — pre-tax IRA balances are irrelevant. The taxable exposure is limited to earnings between contribution and conversion. This makes the mega backdoor available to exactly the population the regular backdoor fails: high earners with large rollover IRAs they can't easily hide from pro-rata.

The mega backdoor by the numbers

What the strategy actually moves, for a high saver whose plan supports it, at 2025 limits:

Twenty years of a $35,000 annual mega backdoor at 7% growth builds roughly $1.4M of Roth — permanently tax-free, RMD-exempt, and inheritable on favorable terms. The strategy's constituency is narrow by construction: people who already max the deferral, have $30K+ of additional annual savings capacity, and work somewhere with the right plan document. For that population, nothing else in the code moves comparable money into tax-free status.

Execution: conversion speed is the whole game

After-tax contributions grow tax-deferred but their earnings convert as taxable income — so the interval between contribution and conversion is a tax leak, and plan mechanics determine its size. The gold standard is automatic in-plan conversion (some payroll systems convert same-day, making the process literally invisible); the workable tier is manual in-plan conversion you trigger quarterly; the clunky tier is in-service distribution to an external Roth IRA, which works but adds paperwork and, in some plans, per-event fees. When enrolling, three questions to benefits: does the plan accept after-tax (non-Roth) contributions, does it offer automatic daily/percentage-based Roth in-plan conversion, and is there a separate election for after-tax contribution percentage in payroll. One operational trap deserves flagging: front-loading after-tax contributions early in the year can crowd out later deferrals if payroll enforces the combined ceiling naively, and in plans with per-paycheck match (no true-up), maxing too fast forfeits match dollars — sequencing deferral-for-match first, after-tax second, is the safe order.

What people get wrong

• Confusing after-tax with Roth 401(k). Roth 401(k) contributions live under the $23,500 deferral limit; after-tax contributions are the separate bucket above it. Plans list them as distinct elections, and the mega backdoor runs entirely on the second.
• Assuming every plan allows it. Roughly half of large-employer plans offer after-tax contributions, and fewer pair them with clean conversion. Small-company plans often can't, because ACP nondiscrimination testing fails without broad participation — a legal constraint, not stinginess.
• Ignoring the earnings between contribution and conversion. A year of unconverted after-tax gains at 10% on $35K is $3,500 of needless ordinary income at conversion. Convert on a schedule, not on memory.
• Doing this before cheaper priorities. The mega backdoor sits behind full match, HSA, deferral max, and (usually) the regular backdoor in the standard order — it's the overflow vessel, not the foundation.

Is it worth it for you? The honest decision tree

The strategy's math is unambiguous; whether it's your math depends on three gates walked in order. Gate one, savings capacity: after maxing the match, HSA, and $23,500 deferral, is there genuinely $10K+ of annual surplus seeking a home? If that surplus is earmarked for a house down payment inside five years, taxable savings' flexibility beats Roth's tax treatment — the mega backdoor is for long-horizon money. Gate two, plan support: the two-feature check with benefits (after-tax contributions + conversion mechanism), plus the ACP-testing reality that even offered features occasionally refund contributions to highly-compensated employees in low-participation plans — a refund is an inconvenience, not a catastrophe, but worth knowing exists. Gate three, the alternative: the comparison isn't Roth-versus-nothing but Roth-versus-taxable-brokerage, where the taxable account's advantages (any-purpose liquidity, capital-gains rates, loss harvesting, basis step-up at death) partially offset the Roth's zero-tax growth. The crossover analysis generally favors the Roth for holding periods past ~10 years and for anyone expecting meaningful dividends or turnover; it favors taxable for near-term goals and for very low-income retirements where capital gains would land in the 0% bracket anyway. Most high savers who clear gates one and two split the surplus rather than choosing — which is also the honest hedge against the strategy's legislative risk.

A worked year, start to finish

Concreteness helps: a 40-year-old earning $220K, plan supports everything. January–December, payroll deferrals of $23,500 pre-tax capture the full $9,000 match by year-end (spread to avoid the no-true-up trap). Simultaneously, an 18% after-tax election contributes $39,600 — but the §415(c) ceiling is $70,000, and $23,500 + $9,000 leaves only $37,500 of room, so payroll caps the after-tax at that figure automatically (good systems) or requires a mid-year election trim (manual ones — a September check catches it). The plan's automatic daily conversion sweeps each after-tax dollar to Roth within the week, so conversion-time earnings round to zero. Year-end result: $70,000 into the plan, $37,500 of it now permanent Roth, tax cost of the conversions approximately nothing, and one Form 1099-R next January reporting the in-plan conversions with a taxable amount near zero — filed, not feared.

The self-employed variant: solo-401(k) mega backdoor

Business owners without employees can build the whole apparatus themselves: a solo-401(k) with after-tax contributions and in-plan conversion written into the plan document reaches the same $70,000 ceiling on self-employment income. The catch is that mainstream free solo-401(k)s (the big brokerages' stock documents) usually omit after-tax provisions — the strategy requires a customized plan document from a specialist provider, typically $500–1,500 to establish plus modest annual fees, which pays for itself in one year at meaningful contribution levels. The compliance load is real but bounded: Form 5500-EZ filing once plan assets exceed $250,000, and honest bookkeeping of the contribution buckets. For a consultant netting $200K+, the solo mega backdoor is frequently the single largest available tax-advantaged savings expansion — and unlike the corporate version, nobody else's participation rate can test it away.

Where the rules come from

The three-bucket structure and combined ceiling are IRC §415(c), with 2025's $70,000 set by Rev. Proc. 2024-40; the deferral limit is §402(g). In-plan Roth conversions were authorized by the Small Business Jobs Act (2010) and expanded to all vested amounts by ATRA (2013); in-service distribution of after-tax amounts with earnings-splitting between Roth and traditional destinations follows IRS Notice 2014-54, the guidance that made the external-rollover variant clean. ACP testing that constrains small plans is §401(m). The strategy survived its closest legislative call in the 2021 Build Back Better drafts, which would have ended after-tax conversions — the provisions died with the bill, a history worth remembering as a reason to use the window while it exists.

Frequently asked questions

Is the mega backdoor Roth still legal?

Yes. Proposed eliminations (notably 2021's Build Back Better drafts) never became law. It remains legal in 2026, though always worth a headline check before large moves — this corner of the code attracts legislative attention.

Mega backdoor vs. just investing in a brokerage account?

After maxing normal tax-advantaged space, Roth conversion of after-tax 401(k) money beats taxable investing on pure tax treatment: zero tax on growth versus annual dividend drag plus capital gains later. Brokerage wins on liquidity — no age-59½ considerations. Most high savers doing this fund both.

Do after-tax contributions reduce my paycheck like regular 401(k) money?

They come from after-tax pay, so a $1,000 contribution costs $1,000 of take-home (versus ~$680 for pre-tax at a 32% bracket). It's a cash-flow-heavy strategy by design — that's who the remaining headroom is for.

Does the mega backdoor affect my regular backdoor Roth?

No — they're independent pipelines. The 401(k)'s after-tax bucket lives outside the IRA aggregation rules, so running both moves $42,000+ per person per year into Roth. The only interaction is budgetary: both consume after-tax savings capacity.

What happens to after-tax 401(k) money if I change jobs before converting?

It rolls out cleanly under Notice 2014-54: after-tax basis to a Roth IRA, its earnings to a traditional IRA (or convert them, paying tax), in one coordinated rollover. Departing employees often use the exit itself as the conversion event — just mind that the earnings-to-traditional-IRA route creates exactly the pre-tax IRA balance that complicates future regular backdoors.