The mega backdoor Roth uses after-tax 401(k) contributions, which sit above the normal deferral limit, and converts them to Roth where growth is tax-free forever. This calculator shows how much room the current-year limits leave you after your own deferrals and your employer’s contribution, and what that room could be worth.

This calculator runs in your browser and needs JavaScript. With scripts off you can still read the explanation below, which walks through the same maths by hand.

Your 401(k) this year

Eligible compensation Salary plus bonus and RSU income counted by your plan.

Your pre-tax or Roth 401(k) contributions Capped automatically at the annual limit plus any catch-up.

Employer contribution as a percentage of pay Enter the effective match, e.g. 50% of 8% is 4%.

Or employer contribution in dollars Overrides the percentage if filled.

Your age at year end Catch-up contributions start at 50; a higher catch-up applies at 60 to 63.

Plan cap on after-tax contributions Only if your plan limits after-tax contributions below the IRS room.

Projection

Years of contributions

Assumed annual return

Tax rate on brokerage gains for comparison Long-term capital gains rate you would pay outside a Roth.

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Running these numbers for a real decision? The calculator shows the mechanics. It cannot see your full return, your other income, or your state’s quirks. Before you act on a large vest, exercise or conversion, have a CPA or a fee-only advisor who works with equity compensation check the figures. Find a fee-only adviser near you or get matched.

How the limits stack

Three separate limits govern a 401(k). The elective deferral limit caps what you put in pre-tax or Roth. The overall Section 415(c) limit caps everything that goes into your account in a year: your deferrals, your employer’s match or profit-sharing, and any after-tax contributions, up to 100% of your pay. Catch-up contributions for people 50 and over sit outside both. The difference between the overall limit and what you and your employer already contribute is the after-tax room this calculator reports.

After-tax contributions are not Roth contributions. They go in with no deduction and their growth is taxable until converted. The mega backdoor works because the plan lets you convert that after-tax money to Roth almost immediately, so there is little growth to tax at conversion and all future growth is tax-free.

Three plan features you need

After-tax contributions allowed. Many large tech employers’ plans include this; many smaller plans do not. Your summary plan description or the recordkeeper’s contribution page will say.
In-plan Roth conversion or in-service withdrawal. Either lets you move the after-tax money to Roth while still employed. Some plans automate it as a daily or per-payroll conversion.
Room under non-discrimination testing. After-tax contributions by highly compensated employees are tested each year. If too few lower-paid employees contribute, part of yours can be refunded in the following spring. Large employers with broad participation rarely fail.

The Amazon 401(k) guide and the Apple benefits guide walk through the mechanics at two employers whose plans are widely reported to allow it; the high-earner planning guide covers where this fits among the other moves.

What the extra Roth space is worth

The projection compounds your annual after-tax room at the return you choose and compares it with the same money in a taxable brokerage account, taxing the growth once at the capital gains rate you enter. Real brokerage drag is somewhat higher because dividends are taxed along the way, and the Roth also escapes state tax and the 3.8% net investment income tax. The figure is conservative.

Worked example

Item	Amount
Employee deferral (at the annual limit)	$24,500
Employer contribution, 4% of $250,000	$10,000
Overall 415(c) limit	$72,000
After-tax room	$37,500

Twenty years of $37,500 at 7% grows to roughly $1.54 million, of which about $790,000 is growth that a brokerage account would eventually tax.

Assumptions and limits

Uses the current-year deferral, catch-up, overall and compensation limits.
Treats the employer contribution as counting fully toward the overall limit, which is how most plans work; forfeiture allocations and true-ups also count.
Assumes the full after-tax amount is converted promptly, so no conversion tax is modelled.
Ignores testing refunds and any plan-specific percentage caps unless you enter a cap.

Frequently asked questions

Is this the same as a backdoor Roth IRA?

No. The backdoor Roth IRA is a non-deductible IRA contribution converted to Roth, limited to the IRA contribution cap and complicated by the pro-rata rule if you have other pre-tax IRA money. The mega backdoor runs inside the 401(k), has far more room, and the pro-rata rule does not apply.

Should I do this before maxing the regular deferral?

Generally no. Capture the full employer match first, then the full pre-tax or Roth deferral, then an HSA if eligible, then the after-tax space.

What if my plan does not allow it?

Then the room does not exist for you. A taxable brokerage account with low-cost index funds is the next-best home for the money, and the fund guide covers what to hold there.