Mega Backdoor Roth can let an eligible saver contribute up to $47,500/year extra into Roth-taxed retirement accounts via after-tax 401(k) + immediate conversion. Estimate your capacity below.
After-tax contribution capacity (2026)
—
Tax-free Roth balance at retirement
—
IRS combined limit (2026)
—
Already used (employee + employer)
—
Your breakdown
Updates live as you type| Input | Value |
|---|
The contribution room hiding above your $24,500
Most high earners think they max out their 401(k) at $24,500 in 2026 and stop. But the IRS sets a larger ceiling on regular employee, employer, and after-tax contributions: $72,000 in 2026 before eligible catch-up contributions. That total counts your own pretax and Roth deferrals, your employer match, and a third bucket many people never use, after tax contributions. The mega backdoor Roth fills that third bucket and then immediately moves the money into Roth treatment, turning what would be a lightly taxed account into one that grows and comes out tax free.
This calculator measures the gap. It takes the $72,000 ceiling, subtracts regular employee deferrals and employer contributions, and shows what room is left for after-tax dollars. That leftover is your mega backdoor capacity. For a 35 year old who has maxed the $24,500 deferral and gets $10,000 in match, the tool reports $37,500 of room.
Two plan features that both have to exist
This strategy is not available to everyone, and there is no workaround if your plan lacks the plumbing. Your 401(k) has to allow after tax contributions, which are different from Roth contributions, and it has to allow either an in plan Roth conversion or in service withdrawals so you can move that money to a Roth IRA. Both pieces are required. Plenty of plans offer one and not the other. IRS Notice 2014-54 explains how pretax and after-tax amounts may be allocated across simultaneous rollover destinations, but your plan document still has to permit the moves. Call your plan administrator and ask both questions before you contribute a dollar.
Stacking $33,500 a year for two decades
Consider a 40 year old who has already used the full $24,500 employee deferral and receives $14,000 in employer contributions. Against the $72,000 ceiling, that leaves $33,500 of after tax room. Suppose they contribute that full amount every year for 20 years, convert it to Roth right away, and earn 7 percent annually. The tool deposits the contribution at the start of each year and compounds, which produces a tax free balance of about $1.47 million.
The chart traces that balance growing across the 20 years. Contributions add a steady block each year while compounding does the heavy lifting toward the end.
Convert fast so earnings stay small
The timing of the conversion matters more than people expect. After tax dollars you contribute are already taxed, so converting your basis is tax free. But any investment gains that accrue while the money sits in the after tax bucket are taxable when you convert. If you let a contribution ride for a year before converting, you owe ordinary income tax on whatever it earned. The fix is to convert immediately, ideally with each paycheck. Many large plans now offer an automatic in plan Roth conversion setting that does this for you the moment the after tax money lands. Turn it on. If your plan only allows quarterly or annual conversions, expect a small taxable amount and report it accordingly.
One judgment call worth flagging: this strategy only makes sense after you have captured your full employer match and maxed your regular deferral. Those come first. The mega backdoor is the next tier up for people who still have cash to invest after the standard buckets are full, which is exactly the audience this tool is built for, high earners at employers with generous plan features.
Can I do the mega backdoor and a regular backdoor Roth IRA in the same year?
Yes, they are separate strategies with separate limits. The backdoor Roth IRA moves up to $7,500 of nondeductible traditional IRA money to a Roth IRA and is governed by the IRA contribution limit. The mega backdoor happens entirely inside your 401(k) and is governed by the $72,000 combined limit before catch-up. Many high earners run both in the same year to push the maximum into Roth accounts. Just watch the pro rata rule on the IRA side, which can make the regular backdoor messy if you hold pretax IRA balances.
Do after tax contributions count against my $7,500 IRA limit?
No. After tax 401(k) contributions are a 401(k) feature and sit under the $72,000 combined plan limit, not the IRA limit. The names sound alike, which trips people up, but they are different accounts with different ceilings. You can contribute the full $7,500 to an IRA and still use your entire mega backdoor capacity inside the 401(k) in the same year.