Mega Backdoor Roth Calculator · Generated September 26, 2026
Mega Backdoor Roth Calculator
The 2026 combined 401(k) limit is $72,000 — far above the $24,500 standard deferral most people think of as the cap. If your plan allows it, the gap between the two can become extra Roth space. See your actual room, and what delaying the Roth conversion costs you.
Does your plan support this?
?
This is the standard 401(k) limit — $24,500 for 2026 — Roth and Traditional combined. Mega backdoor room only opens up once this (plus any employer money) is accounted for.
?
A third bucket — not pre-tax, not Roth. This is the actual mega backdoor move: dollars beyond the standard limit, capped by the combined IRS ceiling.
?
Only the investment growth that accrues before you convert is taxed — the principal was already after-tax money. Converting sooner means less growth has accrued, so less tax.
Your mega backdoor Roth, projected
The cost of waiting to convert?
Same account balance either way — delay doesn't shrink what you have. It just increases the tax bill you'll owe from other funds when you do convert.
?
Same account balance either way — delay doesn't shrink what you have. It just increases the tax bill you'll owe from other funds when you do convert.
| Convert immediately | 3 mo delay | 6 mo delay | 12 mo delay | 24 mo delay |
|---|---|---|---|---|
| $560 | $2,245yours | $3,939 | $7,349 | $14,255 |
Mega backdoor Roth vs. the same dollars in a taxable brokerage
Balance over time
How this works
- 1Max your elective deferral. Contribute the standard 2026 limit — $24,500 — to your 401(k), Roth and Traditional combined. This is a prerequisite, not optional.
- 2Add after-tax contributions. Contribute after-tax dollars — a third bucket, distinct from both pre-tax and Roth — up to the $72,000 combined IRS limit, minus your deferral and any employer contributions.
- 3Convert to Roth. Move the after-tax dollars to Roth via an in-plan conversion. Only the investment growth accrued since contributing is taxed — the principal was already after-tax money, so converting quickly minimizes the tax.
- 4It grows tax-free from here. Once converted, it behaves exactly like any other Roth money — no further tax, ever, on withdrawal.
Example
$24,500/yr deferral + $6,000/yr employer match leaves $41,500/yr of after-tax room. Contributing $20,000/yr of it, converting after 3 months, at 7% for 20 years:
What this doesn't account for
- • Not every plan allows after-tax contributions or in-plan conversions — check yours first.
- • No state taxes, NIIT, or plan-specific rules on partial or automatic conversions.
- • 2026 IRS limits from Notice 2025-67.
Questions
What's actually different about the "after-tax" bucket, versus Roth or Traditional?
It's a third category some 401(k) plans offer on top of the standard limit — contributions are made with already-taxed money (like Roth), but the growth is taxable until converted (unlike Roth). Converting it to Roth is what makes it "mega backdoor."
How is this different from a regular backdoor Roth IRA?
A backdoor Roth IRA moves a few thousand dollars a year through a Traditional IRA into a Roth IRA. A mega backdoor Roth uses 401(k) plan room instead, and can move tens of thousands more per year — the two aren't mutually exclusive.
Once I convert, can I withdraw that money whenever I want?
Not right away, and not as simply as a Roth IRA. Inside the 401(k), converted money follows the same access rules as any Roth 401(k) balance — generally locked until you leave the job, turn 59½, or qualify for a hardship withdrawal. This is exactly why most people roll a mega backdoor Roth conversion into a Roth IRA: once there, the converted principal (not the growth) can be withdrawn penalty-free after its own 5-year clock, even before 59½.
Why does converting quickly matter so much?
It doesn't change your account balance — the money grows at the same rate whether it's pending or converted. It changes your tax bill: only the growth accrued before conversion gets taxed, so a faster conversion means less growth to tax. See the delay comparison above.
How do I know if my plan supports this?
Check your plan's summary description for "after-tax contributions" and "in-plan Roth conversion" (or "in-service withdrawal") — or just ask HR or your plan administrator directly. Most plans don't offer both, which is the most common blocker in practice.
What's the 2026 combined 401(k) limit?
$72,000 (IRS Notice 2025-67) — employee elective deferrals, employer contributions, and after-tax contributions combined. Catch-up contributions sit outside this limit entirely.
Is this financial or tax advice?
No. This calculator uses simplified assumptions — a flat conversion delay, no state taxes or NIIT, no partial-conversion or plan-specific rules — and isn't personalized financial, tax, or legal advice. See our Terms of Use.
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