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Max deferral + match at a $200k salary leaves $37,500 of Roth room most people never use.
The mega backdoor Roth hides in plain sight: the gap between the $24,500 elective deferral limit and the $72,000 annual-additions cap (2026, Notice 2025-73) can be filled with after-tax contributions and converted to Roth — but only if your plan allows both moves, and only the math tells you what it is worth. This calculator computes your exact room, fills the cap to the dollar, and shows the six-figure difference between converting and leaving earnings taxable.
High earners at companies with after-tax 401(k) plans use it to find the number HR never quotes: the exact gap between their deferral-plus-match and the $72,000 annual-additions cap — and what two decades of converting that gap to Roth is worth versus leaving it taxable.
The §415(c) annual-additions limit is $72,000, covering your elective deferrals, employer match and profit sharing, and after-tax contributions combined. Room for after-tax = $72,000 − your deferral − employer money. Example: $200,000 salary, max $24,500 deferral, $10,000 match → $37,500 of room, on top of a possible $8,000 catch-up at 50+ (catch-up is excluded from the cap but cannot be after-tax). Note the pro-rata trap does not apply here — after-tax 401(k) money converts cleanly, unlike IRA basis.
Two, and both must exist in YOUR plan document: after-tax (non-Roth) employee contributions, and either in-plan Roth conversion or in-service distributions of after-tax money to a Roth IRA. Big-tech plans commonly have both; many smaller plans have neither. Ask HR specifically — a plan that merely offers Roth 401(k) deferrals is not enough. And convert promptly: earnings in the after-tax account are taxable at withdrawal, so every month unconverted leaks value.
The conversion-versus-waiting math is dramatic. $37,500 per year for 20 years at 7%: converted to Roth annually it grows to $1,537,331 completely tax-free. Left in the after-tax account, the same contributions grow to the same gross figure but the $787,331 of earnings is taxed — at 24% you keep $1,348,372. That is a $188,959 difference from one checkbox habit. Against a taxable brokerage the Roth path also wins on every year of tax-free compounding; the only real cost is the paperwork.