Mega Backdoor Roth Room Calculator
Find your after-tax 401(k) contribution room for a mega backdoor Roth — the total plan limit minus your elective deferral and employer contributions.
How it works
Room = total plan limit (+ catch-up) − elective deferral − employer contributions, floored at 0
The mega backdoor Roth uses the gap between your regular elective deferral and the much higher overall §415(c) limit on all 401(k) money. You fill that gap with after-tax (non-Roth) contributions, then convert them to Roth — either in-plan or via a rollover — so the growth is tax-free. It only works if your plan explicitly allows both after-tax contributions and conversions, and the space shrinks dollar-for-dollar as your employer match grows.
Worked example
With a $69,000 total limit, a $23,000 deferral and $6,000 of employer contributions, your after-tax room is 69,000 − 23,000 − 6,000 = $40,000 that can be contributed after-tax and converted to Roth.
Frequently asked questions
What is a mega backdoor Roth?
It is a strategy where you make after-tax contributions to a 401(k) beyond the normal deferral limit, then convert them to Roth so future growth is tax-free. It can move tens of thousands of extra dollars into Roth each year if your plan supports it.
Does every 401(k) allow this?
No. Your plan must permit after-tax contributions above the deferral limit and allow either in-plan Roth conversions or in-service withdrawals to a Roth IRA. Many plans do not, so confirm with your provider first.
Does the employer match reduce my room?
Yes. The total limit covers your deferral, employer match and after-tax contributions combined, so a larger match leaves less after-tax room for the mega backdoor Roth.
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Educational estimate using simplified assumptions — not tax, legal or financial advice. Verify limits and rules with the IRS or a qualified advisor.