Free mega backdoor roth calculator
Turn after-tax 401(k) room under the $70,000 limit into tax-free Roth dollars and see the payoff, updated live, as you type.
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Estimates only, based on a constant real return and steady saving. Not financial advice.
Results are estimates. Consult a professional.
What is the mega backdoor Roth?
The mega backdoor Roth lets you move far more than the usual limits into Roth accounts by making after-tax contributions to your 401(k) and then converting them to Roth — either inside the plan (an in-plan Roth conversion) or by rolling them out to a Roth IRA. Where the IRA backdoor is capped at $7,000, the mega backdoor can move tens of thousands of dollars a year. This mega backdoor Roth calculator projects what those after-tax 401(k) contributions grow into once they are converted to Roth.
Unlike the IRA backdoor, the mega backdoor has no income limit at all — but it has a hard requirement most people fail: your employer's 401(k) must allow both after-tax (non-Roth) contributions and either in-service withdrawals or in-plan Roth conversions. Without both features, the strategy is simply unavailable to you.
How the mega backdoor Roth works
- Max your normal deferral. Contribute up to the 2025 elective deferral limit of $23,500 (pre-tax or Roth), plus catch-ups if eligible.
- Add after-tax (non-Roth) contributions. Fill the gap up to the overall §415(c) limit of $70,000, counting your deferral and any employer match.
- Convert the after-tax money to Roth quickly. Use an in-plan Roth conversion or an in-service rollover to a Roth IRA, ideally before the after-tax dollars earn much.
Your mega backdoor headroom under the $70,000 limit
The size of a mega backdoor Roth is set by the overall §415(c) limit — $70,000 in 2025 — which counts everything that goes into your 401(k): your own elective deferral, your employer's match, and your after-tax contributions. Your after-tax headroom is what is left after the first two:
| 2025 limit | Amount | Counts toward §415(c)? |
|---|---|---|
| Elective deferral (pre-tax or Roth) | $23,500 | Yes |
| Catch-up, age 50+ | +$7,500 | On top of $70,000 |
| Super catch-up, ages 60–63 (SECURE 2.0) | +$11,250 | On top of $70,000 |
| Overall §415(c) limit | $70,000 | The total cap |
| After-tax room | $70,000 − deferral − match | The mega backdoor space |
2025 figures published by the IRS. The §415(c) overall limit is the ceiling the mega backdoor fills.
IRS — 401(k) and profit-sharing plan contribution limits (overall §415(c) annual additions limit).How to use the mega backdoor Roth calculator
- Annual after-tax 401(k) contribution. The amount you convert to Roth each year — your §415(c) headroom, often $20,000–$40,000+ depending on your deferral and match.
- Years contributing. How long you run the strategy.
- Expected return. The average annual growth rate for the Roth balance.
A worked example: $30,000 a year for 20 years
Elena's 401(k) supports after-tax contributions and automatic in-plan Roth conversion. After her $23,500 deferral and her match, she has room to add about $30,000 of after-tax money a year, converting each year for 20 years at a 7% return. To reproduce this, set Annual after-tax contribution = 30000, Years = 20, Expected return = 7.
Step 1 — Contribute after-tax and convert each year
Each year $30,000 of after-tax money goes in and is converted to Roth right away, so the taxable earnings on it are negligible. Over 20 years she contributes $600,000.
Step 2 — Let the Roth compound tax-free
Who can use the mega backdoor Roth
The mega backdoor Roth is the most powerful of the Roth strategies but also the most restricted by plan design. You can use it only if all of these are true:
- Your 401(k) allows after-tax (non-Roth) contributions beyond the regular deferral — many plans do not offer this at all.
- Your plan allows in-plan Roth conversions or in-service withdrawals so the after-tax money can be moved to Roth promptly.
- You have headroom under the $70,000 §415(c) limit after your deferral and employer match.
- You can afford to save heavily — the strategy makes sense once you have already maxed your regular deferral and other tax-advantaged space.
Crucially, there is no income limit — high earners shut out of a direct Roth and even out of a clean IRA backdoor can still use the mega backdoor, because everything happens inside the 401(k). If your plan lacks these features, the IRA-based backdoor Roth IRA is the smaller-scale alternative.
Mega backdoor vs backdoor Roth: the differences
| Backdoor Roth IRA | Mega backdoor Roth | |
|---|---|---|
| Account | Traditional IRA → Roth IRA | After-tax 401(k) → Roth |
| 2025 annual ceiling | $7,000 ($8,000 if 50+) | Up to $70,000 minus deferral & match |
| Income limit | None on the conversion | None |
| Requires special plan? | No — any IRA | Yes — after-tax + in-plan/in-service conversion |
| Pro-rata concern | Across ALL your IRAs | Inside the 401(k) (basis vs earnings) |
| Reported on | Form 8606 | Plan/1099-R; no Form 8606 for the 401(k) step |
Both build tax-free Roth dollars for high earners, but through different accounts, ceilings, and rules.
Mega backdoor Roth gotchas
- Assuming your plan offers it. Most do not. Confirm with your plan administrator that after-tax contributions and in-plan Roth conversion (or in-service distribution) are both available before you count on it.
- Letting after-tax money grow before converting. Earnings on after-tax contributions are taxable when converted. Automatic same-day conversion keeps the taxable piece near zero.
- Confusing after-tax with Roth deferrals. After-tax (non-Roth) contributions are a separate bucket from your Roth 401(k) deferral and have their own plan rules.
- Overshooting the $70,000 limit. Your deferral and employer match eat into the §415(c) cap first — exceeding it triggers correction headaches.
- Forgetting catch-ups sit on top. If you are 50+ or 60–63, your overall cap is higher than $70,000, giving extra after-tax room.
Accuracy, assumptions, and sources
This mega backdoor Roth calculator projects the future value of equal annual after-tax contributions that are converted to Roth, at a fixed return. It assumes prompt conversion with negligible taxable earnings and does not model your plan's specific limits, the §415(c) cap, taxes, or fees. Plan features and limits vary — confirm your headroom and conversion options with your plan administrator, and treat the result as a planning estimate, not tax advice. Verify current limits on IRS.gov.
IRS — 401(k) limit increases to $23,500 for 2025 (elective deferral and catch-up amounts).IRS — Rollovers of after-tax contributions in retirement plans.Frequently asked questions about the free mega backdoor roth calculator
About this mega backdoor Roth calculator
This calculator projects the tax-free growth of after-tax 401(k) contributions converted to Roth — the mega backdoor strategy, which can move far more than an IRA backdoor but requires an employer plan that supports after-tax contributions and in-plan or in-service Roth conversion. Your room is set by the 2025 $70,000 overall §415(c) limit minus your deferral and match.
See it with our other retirement calculators, or open the full calculator directory for everything else. Figures are 2025 IRS limits; results are planning estimates, not tax advice.