Comprehensive Guide
Learn more in our Investing Guide.
How it works
The mega backdoor Roth exploits the gap between the employee DEFERRAL limit everyone knows and the far larger overall 415(c) ceiling that includes employer money and after-tax contributions. Where a plan permits after-tax contributions plus either in-plan Roth conversion or immediate rollovers, salary beyond your normal deferral can flow into the plan, get taxed once on the way in, and land in your Roth stack — potentially tens of thousands of extra tax-advantaged dollars a year for high savers. Room equals the overall limit minus deferrals, minus employer contributions, minus after-tax money already in; this checker computes the remainder in yearly and monthly terms and translates the effort into total-savings-rate percentages. Three plan features must coexist — after-tax contributions allowed, a conversion mechanism, and no lower internal cap — so verifying the summary plan description comes before any payroll change. Limits shown here are user-entered illustrations; pull the live figures each January. Earnings that accumulate on unconverted after-tax money come back out taxable, which is why automation matters as much as eligibility.Formula
Room = overall limit − employee deferral − employer money − after-tax contributed | Monthly pace = room ÷ months left
Tips
- Confirm three plan features: after-tax allowed, in-plan conversion or instant rollover, no internal cap.
- Automate conversions at payroll frequency so earnings never build a taxable tail.
- Max the regular deferral FIRST — it has better tax treatment than after-tax money.
- Recalculate room each January; limits and matches reset and rarely match your assumptions.
- Watch true-up provisions — front-loading deferrals can forfeit match without them.
Frequently asked questions
What is the mega backdoor Roth contribution limit?
It is whatever remains under the overall 415(c) limit — often around $72,000 including everything — after employee deferrals, employer money and after-tax amounts. On the defaults used here ($24,500 + $6,000 + $10,000 in), about $31,500 of room remains for additional after-tax dollars.
How is the mega backdoor Roth different from a regular backdoor Roth?
A regular backdoor Roth sneaks high earners through an IRA with nondeductible contributions and conversion; the mega version lives inside a workplace 401(k) using after-tax contributions well above the deferral limit. The mega path needs specific plan features; the regular path mostly needs no conflicting IRA balance.
Do after-tax 401(k) contributions get taxed twice?
Not if converted promptly. You pay income tax as wages anyway; anything earned BEFORE conversion comes back out taxable. A 5% earnings tail on $30,000 of after-tax money means $1,500 taxed twice — same-payroll conversions keep that taxable tail at essentially zero.
Can my employer block the mega backdoor Roth?
Yes, silently. If the plan lacks after-tax contributions or a conversion mechanism, or caps total contributions below the roughly $72,000 legal ceiling, the strategy simply does not exist there. Read the summary plan description or ask the administrator before touching payroll elections.
Is mega backdoor Roth money worth it versus taxable investing?
Generally yes by a wide margin: the same dollars compound without annual dividend and capital-gains taxes and emerge qualified-tax-free, where a taxable account leaks yearly. Liquidity differs — Roth earnings lock until 59½ under most rules — so keep nearer-term goals outside the plan.