Mega backdoor Roth calculator
The mega-backdoor Roth is the largest tax-free savings opportunity in the US code that most people never use: after-tax contributions to a 401(k), immediately converted to Roth, on top of your regular deferral. The 2026 elective limit is $24,500, but the total that can go into a 401(k) from all sources is $72,000 — and the gap between those two numbers is space almost nobody fills. Coastline computes your actual room under §415(c) and compounds it through a full projection.
Your room, and the counter-intuitive part
The room is $72,000 minus your own elective deferrals minus everything your employer puts in. Which means a more generous employer match shrinks your mega-backdoor space — the two share one limit:
| Salary | Your deferral | Employer match | After-tax room | What binds it |
|---|---|---|---|---|
| $150,000 | $24,500 | $6,000 | $35,171 | capped by available cash (§415(c) would allow $41,500) |
| $200,000 | $24,500 | $8,000 | $39,500 | §415(c) limit |
| $250,000 | $24,500 | $10,000 | $37,500 | §415(c) limit |
| $400,000 | $24,500 | $16,000 | $31,500 | §415(c) limit |
| $250,000 | $24,500 | no match | $47,500 | §415(c) limit |
| $250,000 | none | $10,000 | $62,000 | §415(c) limit — but you gave up the deduction |
Two details the arithmetic hides. Catch-up contributions from 50 sit on top of §415(c) rather than inside it, so they don't consume this room. And at lower salaries the binding constraint often isn't the tax code at all — it's cash: the $150,000 row is capped at $35,171 because that is what the household actually had left after tax, spending, and the regular deferral, not because §415(c) said so (it would have allowed $41,500). A projection that ignores cash flow will happily tell you to contribute money you don't have.
What the space compounds to
Filling $37,500 of after-tax room every year, invested at 7% nominal, shown in today's dollars at 65:
| Start at | Years of contributions | Total contributed | Roth balance at 65 |
|---|---|---|---|
| 30 | 36 | $1,350,000 | $2,840,512 |
| 35 | 31 | $1,162,500 | $2,180,324 |
| 40 | 26 | $975,000 | $1,634,648 |
| 45 | 21 | $787,500 | $1,183,621 |
| 50 | 16 | $600,000 | $810,826 |
The honest accounting: why your net worth doesn't move
Here is something no other mega-backdoor calculator will tell you. In the projection above, total net worth at 65 is identical with and without the strategy — $8,064,662 either way. The mega-backdoor is after-tax money: it doesn't reduce your taxable income, so nothing is saved today. All it does is move $2,180,324 of your wealth out of a taxable brokerage account ($5,431,535 down to $3,251,211) and into a Roth.
The entire benefit is what happens after 65: Roth withdrawals are tax-free and never required, while the brokerage account owes capital-gains tax on every dollar of gain you realise. So the payoff only appears in a plan that actually draws the money down. Run the same household to 95 in a state that taxes capital gains as ordinary income, and tax across the retirement years falls from $1,256,825 to $790,613 — $466,212 less, in today's dollars. Tax across the working years is $1,943,119 either way, unchanged to the dollar, exactly as you would expect from a contribution that never touched your AGI.
Two caveats on that figure, both in your favour and against it. The projection does not tax dividends or annual gains in a taxable account, only realised gains — so it understates the real-world advantage of sheltering the money. But it also means the advantage is smaller in a no-income-tax state, and smaller again for anyone who never spends the taxable account down and passes it on with a stepped-up basis.
Before you try it
- Your plan has to allow two things: after-tax (not Roth) contributions, and either in-plan Roth conversions or in-service withdrawals. Many plans allow neither, and there is no workaround.
- Convert immediately. Any growth between the after-tax contribution and the conversion is taxable on conversion. Same-day or automatic conversion keeps the taxable amount at zero.
- Watch the nondiscrimination tests. In plans where few employees make after-tax contributions, refunds to highly-compensated employees are common.
- Fill the free money first. The match, then the $24,500 deferral (or Roth deferral), then an HSA, then a Roth IRA, and the mega-backdoor after that — it is the last stop, not the first.
Why Coastline's version is different
Most free calculators hand you a single headline number with the assumptions hidden. Coastline is built the opposite way:
- Real tax math, not a flat rate. It applies federal and state income tax, long-term capital-gains rules, and account-by-account treatment (taxable, traditional, Roth) — in both your working years and retirement.
- The whole journey. It models accumulation (saving and investing) and drawdown (spending it down), so you see how the plan connects end to end, not just one half.
- The math is shown. Every figure has a click-through breakdown of how it was computed — a level of transparency even paid tools rarely expose.
- US and Canada. It natively handles Canadian plans (RRSP, TFSA, CPP, OAS) alongside US accounts, which most calculators ignore.
- Free, no signup. No account, no email, no account-linking. Your inputs run the projection and are then discarded.
Educational, not tax advice. 2026 limits: $24,500 elective deferral, $8,000 catch-up from 50 (which sits on top of §415(c)), $72,000 total additions per employer plan. Figures assume a single filer in a no-income-tax state (the drawdown comparison uses a state that taxes capital gains as ordinary income), 7% nominal return, 3% inflation, contributions clamped to the cash the household actually has, and are shown in today's dollars. §415(c) applies per employer plan, so a genuine side business with its own solo 401(k) has its own separate limit. Confirm your plan's rules with your provider.
Common questions
How much can I contribute to a mega backdoor Roth in 2026?
The §415(c) total-additions limit of $72,000 per employer plan, minus your own elective deferrals and minus everything your employer contributes. On a $250,000 salary with the full $24,500 deferral and a 4% match ($10,000), that leaves $37,500. With no employer match the same person would have $47,500 of room — a bigger match genuinely shrinks the space, because they share one limit.
Is the mega backdoor Roth worth it?
It is worth it if you have already filled the match, your regular deferral, an HSA, and a Roth IRA, and still have cash to invest — because the alternative is a taxable brokerage account. It saves nothing today: the contribution is after-tax, so net worth at 65 is the same either way ($8,064,662 in this projection). The gain is that $2,180,324 of it is tax-free and never subject to required distributions, which in a drawdown scenario cut lifetime retirement tax by $466,212 here.
Does my 401(k) plan allow the mega backdoor Roth?
Many do not. It requires the plan to accept after-tax contributions (which are different from Roth 401(k) contributions) and to permit either in-plan Roth conversions or in-service withdrawals so the money can be moved before it grows. Ask your provider for both specifically. Plans also run nondiscrimination tests that can force refunds to highly-compensated employees when few others participate.
What is the difference between a mega backdoor Roth and a backdoor Roth IRA?
Size and vehicle. A backdoor Roth IRA moves the $7,500 IRA contribution into a Roth for people whose income is too high to contribute directly, and it is affected by the pro-rata rule across your IRAs. The mega backdoor happens inside a 401(k), is not affected by IRA balances, and is far larger — $37,500 a year in this example. Most people who can do both, should.