The mega backdoor Roth uses a much larger pool of contribution room than the standard backdoor Roth IRA — the space between the regular 401(k) deferral limit and the total combined-contribution ceiling under IRC §415(c).
How it works
The §415(c) limit caps everything flowing into a defined-contribution plan — employee deferrals, employer contributions, and after-tax contributions — at one combined figure. Subtracting employee deferrals and employer contributions from that total reveals how much after-tax room is left, if the plan allows it.
What this does not include
Only a plan that both permits after-tax contributions beyond the regular deferral limit and allows in-plan Roth conversions or in-service withdrawals can actually use this room — most 401(k) plans don’t offer both features, and this calculator doesn’t check plan eligibility, only the dollar math.
How to use this calculator
- Enter the current year’s §415(c) total limit.
- Enter your own employee deferral and any employer match or profit sharing.
Frequently asked questions
Does my 401(k) plan allow this?
Not automatically — check with your plan administrator whether it permits after-tax contributions above the regular deferral limit and in-plan Roth conversions or in-service withdrawals.
How is this different from a regular backdoor Roth IRA?
The IRA version covers a few thousand dollars of IRA contribution room; this covers the much larger 401(k) after-tax space, often tens of thousands of dollars, bounded by the §415(c) total limit instead of the IRA limit.
What counts toward the §415(c) limit?
Employee deferrals, employer matching and profit-sharing contributions, and after-tax contributions all count toward the one combined total.
