Calculator
Mega-Backdoor Roth Room
It is a subtraction — the annual additions limit, less your deferrals, less what your employer puts in — and it is got wrong constantly, because the catch-up contribution sits outside that limit rather than under it. Subtracting it costs someone over 50 up to $11,250 of room they actually had. This does the subtraction properly, checks the two plan features the strategy depends on, and prices what a converted dollar is worth against the same dollar in a brokerage account.
Uses published 2026 federal figures
Your after-tax room · 2026
Enter your compensation, your deferrals and what your employer puts in, then press Calculate.
This plan year
Salary deferrals are what you put in from your own pay, pre-tax and Roth combined — capped at $24,500 for 2026. Do not include catch-up here; it has its own box.
Employer contributions are the match plus any profit-sharing or non-elective contribution. If you only know the formula, work out what it will pay by year end — the match uses up the same limit your after-tax money is trying to fill.
Compensation matters because the limit is the lesser of $72,000 and 100% of your pay.
Sets your catch-up tier
Pre-tax and Roth together. Capped at $24,500.
Match plus profit sharing, for the whole year
Only from 50. Does not use up any of the limit.
What your plan allows
These two features decide whether the strategy exists for you at all, and neither is guaranteed. Ask your plan administrator for the summary plan description and look for “after-tax contributions” — which is a different thing from “Roth contributions”, and the confusion between the two is why people are told no when the answer is yes.
The second feature is the conversion: an in-plan Roth conversion or an in-service withdrawal. Without it the money still grows tax-deferred, but every dollar it earns before you leave is taxable when it is finally converted.
A separate bucket from pre-tax and Roth deferrals. Roughly half of large plans offer it; most small ones do not.
An in-plan Roth conversion or an in-service withdrawal to a Roth IRA, while you still work there. This is what makes the growth tax-free rather than merely deferred.
A share of pay, if your plan sets one. Leave at 0 if it doesn't, or you don't know.
Over $150,000 forces any catch-up to be Roth from 2026.
What converting it is worth
Contributing after-tax money is not itself interesting — those dollars are taxed on the way in either way. What is interesting is the conversion: once the money is in a Roth, everything it earns from then on is never taxed again.
Taxable drag is the bite a brokerage account takes each year for dividends and fund distributions it cannot defer. Around 0.4–0.6% is typical for a broad index fund; higher for anything that turns over more.
Per year
Annual, on a brokerage account
On the gain at the end
For educational purposes only. Results are estimates and do not constitute financial, tax, or legal advice. Consult a qualified professional before making any financial decisions.
How to use it
The hard input is employer contributions. Your match uses up the same limit your after-tax money is trying to fill, so if you only know the formula — 50% of the first 6%, say — work out what it will actually pay by year end and put that in. Include any profit-sharing or non-elective contribution too.
Put your catch-up in its own box, not with your deferrals. It sits outside the annual additions limit, so mixing the two understates your room by up to $11,250.
If you don't know whether your plan has the two features, leave both switches on. You'll see the number the strategy is worth, which is what tells you whether it's worth the phone call to find out.
Press Example to load a worked case: $250,000 of pay, a maxed deferral, and a $12,000 match.
The mistake this exists to prevent
There are three limits in play and they do not nest the way people assume.
§402(g) caps what you can defer from your own salary at $24,500. §415(c) caps everything that can land in the account — $72,000 — including your deferrals, the employer's money, and any after-tax contributions.
The catch-up — $8,000 from 50, and $11,250 in the four years you are 60 to 63 — sits outside both. It is genuinely extra.
So a 55-year-old who subtracts their catch-up from the §415(c) limit before working out their after-tax room comes up $8,000 short, and simply never contributes money they were entitled to contribute. The stacked bar above deliberately leaves catch-up off, because it is not competing for that space.
What to ask your plan administrator
Two features, and the wording matters more than it should. Ask for the summary plan description and look for both:
1. Does the plan accept after-tax contributions? Not Roth contributions — those are a different bucket with a different limit, and confusing the two is the single most common reason somebody is told no when the answer is yes. Roughly half of large plans offer after-tax; most small ones do not.
2. Does it allow in-plan Roth conversion, or in-service withdrawal? This is what turns tax-deferred growth into tax-free growth. Without it the contributions still work, but the earnings that accrue before you leave are taxable when they are finally converted.
If both are yes, ask how often you can convert. Some plans do it automatically on every payroll, which is ideal — there are no earnings to tax if the money is converted the day it arrives.
Assumptions and limits
- Every limit is the published 2026 figure from IRS Notice 2025-67. All of them change annually, and this page is date-stamped for that reason.
- The annual additions limit applies per employer, so two unrelated jobs give you two of them — but the salary deferral limit is shared across every plan you are in.
- Not modeled: non-discrimination testing, which can force a plan to refund after-tax contributions made by highly compensated employees after year end; the pro-rata rule where an after-tax balance has accrued earnings before conversion; and state tax of any kind.
- The projection compares the same dollars in a Roth against a brokerage account. It assumes a steady return and a steady drag, which no real account has.
- An estimate for planning. Not tax advice, and not a substitute for someone reading your actual plan documents.