Mega Backdoor Roth Calculator: Your After-Tax Room Under the $72,000 Limit

Updated August 2, 2026. Quick answer: the mega backdoor fills the gap between what you and your employer put into your 401(k) and the total the law allows, which is $72,000 for 2026. Someone deferring the full $24,500 with $12,000 of employer match has $35,500 of after-tax room. But it only exists if your plan offers two specific features, and many do not — that is the first thing to check, before any arithmetic.

The plan-feature gate, first

Both of these are optional provisions your employer chose or did not choose. They are not rights, and one without the other is close to useless:

1. After-tax contributions — a third contribution type, distinct from pre-tax and from Roth deferrals. Many plans do not offer it at all.
2. In-plan Roth conversion, or in-service withdrawal — a way to move those after-tax dollars into Roth treatment promptly. Without it the money sits as after-tax inside the plan and its earnings grow taxable, which is a materially worse deal.

Ask your plan administrator both questions in those words. If either answer is no, stop here — nothing below is available to you, and no amount of planning changes that.

The calculator

The three limits people confuse

The mega backdoor lives entirely in the gap between the first row and the second. That is the whole idea, and it is why a generous employer match — which is good news — shrinks the room available.

Catch-up contributions do not count against it

This surprises people, and it is worth stating from the statute. Catch-up contributions “shall not, with respect to the year in which the contribution is made … be subject to any otherwise applicable limitation contained in sections 401(a)(30), 402(h), 403(b), 408, 415(c), and 457(b)(2)” (IRC § 414(v)(3)(A)).

So the $72,000 is not your ceiling if you are 50 or older. Add $8,000, or $11,250 in the years you are 60 to 63, entirely on top. Why the catch-up sits outside the annual additions limit works through what that means in practice, and what the whole catch-up stack is worth prices it.

Why the conversion timing matters so much

After-tax dollars are not Roth dollars. They are already-taxed principal sitting in a plan whose earnings are taxable when withdrawn. Converting promptly — ideally automatically, if your plan supports it — means almost nothing has grown yet, so almost nothing is taxable on conversion.

Leave the money as after-tax for a year and the growth is taxable when you convert it. Some plans offer automatic in-plan conversion on every after-tax payroll deposit, which is the ideal setup and worth asking for by name.

How this differs from the ordinary backdoor Roth

Different account, different rule, different order of magnitude. The ordinary backdoor moves an IRA contribution into a Roth IRA and is governed by the IRA aggregation rule — which is why clearing pre-tax IRA balances first matters there. The mega backdoor happens inside your 401(k), so IRA balances are irrelevant to it, and the amounts are far larger. Many high earners do both in the same year: they are not alternatives.

The pro-rata calculator covers the IRA side.

If you are self-employed

A solo 401(k) can be written to include both features, because you control the plan document — which makes the mega backdoor considerably more accessible to the self-employed than to most employees. That is a plan-design question rather than a contribution question, and it is covered separately in the self-employed retirement material.

2026 limits from IRS Notice 2025-67, downloaded from irs.gov and extracted locally; the catch-up exclusion from IRC § 414(v)(3)(A). The notice’s deferral and catch-up figures independently reproduce a separate staged extraction from the IRS announcement, which is how they were verified. Limits are indexed and change annually. General information, not tax advice.