Mega backdoor Roth calculator

How the mega backdoor Roth works

There are two separate 401(k) limits each year. The first is the employee deferral limit - the most you can put in as traditional or Roth contributions from your paycheck. The second, much larger limit is the total additions limit (Internal Revenue Code section 415(c)), which counts everything that goes into the account: your deferral, your employer’s match, and any after-tax contributions.

The gap between those two limits is the opportunity. If your plan allows it, you can fill that gap with after-tax dollars and then convert them to Roth - a mega backdoor Roth - moving far more than the normal limit into an account that grows and is withdrawn tax-free.

What this calculator estimates

Enter your own 401(k) deferral so far this year, your employer’s contributions, and whether you are 50 or older. The tool subtracts both from the total additions limit and shows the after-tax room left for a mega backdoor Roth.

Before you rely on the number

The room is only usable if your plan supports two features: after-tax contributions, and either in-plan Roth conversions or in-service withdrawals. Many employer plans support neither, in which case the strategy is not available regardless of the room. Call your plan administrator and ask about both before you change your contribution elections.

If you have maxed your deferral and still have a high savings rate, the mega backdoor Roth is often the single largest additional tax-advantaged bucket available - which is why it is so popular with high earners chasing financial independence.