Mega Backdoor Roth Calculator

Most people treat "maxing the 401(k)" and the elective deferral limit as the same thing: $24,500 in 2026, and done. But the deferral limit is only the employee-contribution ceiling. The plan itself can accept annual additions up to $72,000 under section 415(c) — a figure that counts deferrals, employer contributions and a third category most plans never mention: after-tax (non-Roth) employee contributions.

The mega backdoor Roth lives in the gap. Contribute after-tax dollars into that remaining space, convert them to Roth promptly, and money that could never have entered a Roth IRA directly — the income limits saw to that — compounds tax-free for decades. For someone already deferring the maximum, it is the largest additional tax-advantaged space available in the American system, refreshed every January.

The arithmetic has one counterintuitive corner: employer generosity shrinks it. Match and profit sharing come out of the same $72,000 ceiling, so a plan contributing $40,000 of employer money leaves only $7,500 of after-tax space where a stingier plan would leave $35,500. Catch-up contributions, by contrast, ride above the ceiling entirely — the age-50 catch-up neither uses nor loses any of the space.

None of this exists unless the plan document says so, twice. The plan must accept after-tax contributions — a provision distinct from Roth deferrals, and absent from most plans — and it must offer a route out of the after-tax bucket: in-plan Roth conversion, or in-service distribution to a Roth IRA. Large tech and professional-services employers increasingly offer both with an automatic-sweep election; most small plans offer neither. The summary plan description answers in ten minutes, and requesting the features is how they spread.

The conversion itself rests on Notice 2014-54, which settled that a distribution from the after-tax subaccount can send the basis to a Roth account and any earnings wherever directed. That makes the conversion clean in a way the ordinary backdoor Roth is not: the IRA pro-rata rule — the trap that catches backdoor-Roth users with existing deferred IRA balances — simply does not apply, because a 401(k) subaccount is never aggregated with IRAs.

What is taxable is any earnings accrued between contribution and conversion, at ordinary rates. Convert annually by hand and half a year of growth is taxable on average; convert by automatic sweep and the figure is zero. The difference is small in dollars and large in paperwork, and it is the reason the sweep election is the first box to tick where it exists.

Against the alternative — the same dollars in a taxable brokerage — a year of mega backdoor space run for twenty years at 7% is worth roughly $24,000 of extra after-tax wealth, and that is with a taxable model flattered by ignoring annual dividend drag. The advantage is the entire tax on decades of growth, and it compounds across years: ten years of contributions is ten of these gaps, each running its own clock.

Two 2026 footnotes complete the picture. The Roth catch-up mandate is now operating: anyone whose prior-year FICA wages from the employer exceeded $150,000 must make catch-up contributions as designated Roth under section 414(v)(7)(A). It does not touch the mega backdoor mechanics, but it lands on the same payslips and generates the same HR queries. And the annual-additions ceiling now moves in $2,000 steps — $70,000 to $72,000 this year — so the space grows a little most Januaries without anyone doing anything.

The honest caveats are short. Money converted lives under Roth rules, including the five-year clocks on conversions for under-59½ access. The strategy consumes cash flow that a taxable account would leave accessible without qualification. And highly compensated employees in plans with few other after-tax contributors occasionally see contributions returned when the plan fails its ACP nondiscrimination test — a plan-level fact worth asking about in the same call that confirms the features exist.