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The IRS lets you put up to $70,000 a year into a single 401(k) account in 2025 — counting your contributions, employer contributions, and after-tax contributions combined. Most people only fill the first $23,500 bucket. The Mega Backdoor Roth is the technique that fills the rest of the bucket and immediately converts it to Roth, where it grows tax-free forever. Done annually for 10 years it puts an extra $400,000 to $500,000 into Roth that you would not otherwise have.
Key takeaways
- 2025 401(k) total annual addition limit: $70,000 (employee + employer + after-tax). 2026 limit will be slightly higher.
- The regular employee deferral cap of $23,500 is only the first bucket. After-tax contributions can fill the gap.
- You need two plan features: after-tax contributions allowed, and either in-plan Roth conversion or in-service withdrawal to a Roth IRA.
- Typical Mega Backdoor capacity for a $200k earner with a 6% match: about $40,500 per year.
- Check your Summary Plan Description (SPD) for the phrase "after-tax" — not "Roth," which is different.
How the $70,000 ceiling works
Section 415(c) of the Internal Revenue Code caps the total annual additions to a defined-contribution plan. For 2025 that cap is $70,000 (or 100% of compensation, whichever is lower). It applies to the sum of three buckets:
- Employee elective deferrals — pre-tax or Roth, capped at $23,500 in 2025 (plus catch-up if 50+).
- Employer contributions — match, profit sharing, non-elective. Whatever your company puts in.
- Employee after-tax contributions — the bucket almost nobody uses, capped only by the $70,000 total minus the other two.
| Bucket | 2025 limit | Tax treatment of contribution | Tax on growth |
|---|---|---|---|
| Employee deferral (Traditional) | $23,500 | Deductible | Deferred |
| Employee deferral (Roth) | $23,500 | After-tax | Tax-free |
| Employer match / profit share | No separate cap | Pre-tax | Deferred |
| Employee after-tax | $70,000 minus the above | After-tax | Taxed unless converted |
A worked example: $200k earner at a large tech company
You earn $200,000. Your company offers a 6% match. You contribute the full $23,500 deferral.
- Employee deferral: $23,500
- Employer match (6% of $200k): $12,000
- Running total: $35,500
- Remaining capacity to $70,000: $34,500 in after-tax contributions
That $34,500 gets immediately converted to Roth — either via in-plan conversion or by rolling to a Roth IRA. Result: an extra $34,500 of Roth space per year that nobody else even noticed.
Why the conversion step matters
After-tax contributions on their own are not great. The contributions come out tax-free in retirement (you already paid tax on the dollars), but the growth on those contributions is taxed as ordinary income at withdrawal. That makes after-tax contributions worse than a regular taxable brokerage account in many cases, because long-term capital gains rates are lower than ordinary income rates.
The conversion turns the after-tax contribution into Roth. Now both the contribution and the growth are tax-free forever. This conversion is what makes the strategy worth doing. The conversion needs to happen fast — ideally the same paycheck — so that the after-tax money has no time to generate growth that would itself be taxable on conversion.
Two ways to convert
In-plan Roth conversion
The plan converts after-tax contributions to the Roth side of the 401(k) automatically, typically each pay period. You sign up once, and the plan handles the mechanics. This is the cleanest version. Companies like Microsoft, Google, Meta, Amazon, and many others offer it.
In-service withdrawal to Roth IRA
If in-plan conversion is not available, the plan may allow you to withdraw after-tax contributions while still employed and roll them to an outside Roth IRA. You request the rollover quarterly or annually, the plan cuts a check or direct-transfers to Fidelity, Schwab, or Vanguard, and the after-tax portion goes to Roth IRA while any earnings on it go to a Traditional IRA.
Both work. In-plan is more automated; in-service gives you the wider investment menu of a Roth IRA at Fidelity or another major broker.
How to check if your plan allows it
You need both features. Either is useless without the other. Look for the following:
- Read the Summary Plan Description (SPD). Search for "after-tax" contributions specifically. Do not confuse this with Roth contributions. "Roth 401(k)" is the employee deferral bucket; "after-tax" is the separate third bucket.
- Check the contribution election page in your benefits portal. If you see three sliders or three boxes — Pre-tax, Roth, After-tax — your plan has the first feature.
- Search the SPD for "in-plan Roth conversion" or "in-service withdrawal." One of these phrases needs to appear.
- Call the plan administrator. Ask: "Does the plan allow after-tax contributions, and does it allow automatic in-plan Roth conversion of those contributions?" The phone rep should know.
If the plan has after-tax contributions but no conversion mechanism, the strategy is significantly weaker. You can still do an out-of-plan rollover when you eventually leave the employer, but the growth between contribution and rollover gets taxed.
How to size your contribution
The math is mechanical:
| Input | Example |
|---|---|
| Annual addition limit (2025) | $70,000 |
| Minus your elective deferral | − $23,500 |
| Minus expected employer contributions | − $12,000 |
| Equals after-tax contribution cap | = $34,500 |
| Divided by remaining pay periods | ÷ 24 (semi-monthly) |
| After-tax contribution per paycheck | = $1,438 |
The true-up trap
If you frontload your regular $23,500 deferral and hit the cap in July, your employer may stop matching for the rest of the year unless the plan has a "true-up" provision. Check your SPD. If there is no true-up, spread the elective deferral evenly across all pay periods.
Mega Backdoor Roth vs other priorities
The Mega Backdoor only makes sense after you have done the basics. The priority order for a high earner:
- 401(k) up to employer match — always.
- HSA to the max if on an HDHP.
- Regular 401(k) elective deferral to $23,500.
- Backdoor Roth IRA ($7,000 in 2025).
- Mega Backdoor Roth — fill the rest of the $70,000 bucket.
- Taxable brokerage.
Why is the Mega Backdoor below the regular Backdoor Roth? Because the regular Backdoor Roth uses dollars that would have gone to a taxable brokerage account. The Mega Backdoor uses dollars that would also have gone to a taxable brokerage. Both turn taxable dollars into Roth dollars; the regular Backdoor is just easier and has no plan-design risk.
Cash flow reality check
Putting $34,500 of after-tax dollars into a 401(k) requires after-tax cash flow of $34,500. On a $200,000 salary, after federal tax, state tax, FICA, regular 401(k) deferral, and basic living expenses, most households have $25,000 to $50,000 of discretionary cash flow. The Mega Backdoor will eat all of it.
For most high earners this strategy gets unlocked only after the mortgage is comfortable, the emergency fund is full, the kids are in regular schools, and lifestyle is settled. It is a strategy for the "too much income for my expenses" problem, not a strategy for someone running tight.
What can go wrong
- Plan does not allow it. Most small employer plans do not. About 60% of large company plans do.
- You leave employment mid-year. Unconverted after-tax balance gets rolled out — the contribution to Roth IRA, the earnings to Traditional IRA. Manageable but messier.
- Highly compensated employee (HCE) refunds. If the plan fails non-discrimination testing, after-tax contributions can be partially refunded. Rare in large plans with safe-harbor design.
- You miscount the limit. The $70,000 includes employer contributions. If your match is 8% on a $250k salary, you have less after-tax room than you think.
- You forget to enroll in conversion. Contributing after-tax without conversion is worse than a taxable account because of ordinary income tax on growth.
Related reading
FAQ
What is the difference between a Roth 401(k) and after-tax contributions?
A Roth 401(k) contribution is an employee elective deferral, capped at $23,500 in 2025. After-tax contributions are a separate bucket that sits on top of that, allowed to fill the $70,000 total. Growth on Roth 401(k) is tax-free; growth on plain after-tax is taxed unless converted.
Do all 401(k) plans allow the Mega Backdoor Roth?
No. Roughly 40-60% of large employer plans offer it. Most small employer plans do not. Read your Summary Plan Description or call the plan administrator.
What is the 2025 Mega Backdoor Roth limit?
$70,000 total annual addition limit, minus your elective deferral ($23,500) and minus employer contributions. The after-tax slot is whatever is left, typically $25,000 to $46,500.
Can I do the Mega Backdoor and the regular Backdoor Roth in the same year?
Yes. They are independent. Most high earners do both: $7,000 via regular Backdoor Roth, plus $25,000-$45,000 via Mega Backdoor Roth.
What happens to the after-tax money if I leave my job?
You roll it out. The after-tax basis goes to a Roth IRA tax-free. Any earnings on the after-tax money that have not been converted go to a Traditional IRA. Use a direct rollover to avoid withholding.
Is the Mega Backdoor Roth going to be eliminated?It has been on the legislative chopping block several times — Build Back Better in 2021 would have killed it. It survived. The risk remains, but as of 2025 the strategy is fully legal.Do I pay tax when I do the in-plan Roth conversion?
You pay tax only on the growth between contribution and conversion. If the conversion happens the same pay period as the contribution, the growth is essentially zero. This is why automatic same-paycheck conversion is the gold standard.
Can I invest the after-tax money before converting?
Technically yes, but do not. Any growth before conversion is taxable on conversion. Convert immediately, then invest the Roth dollars in the same fund you would have chosen anyway.
Bottom line
If you earn enough to max your 401(k), max an HSA, do a Backdoor Roth IRA, and still have money left over, the Mega Backdoor Roth is the next tax shelter in line. Check your Summary Plan Description for the words "after-tax" and either "in-plan Roth conversion" or "in-service withdrawal." If both appear, set up automatic after-tax contributions and automatic conversion, then forget about it. Ten years of this and you have roughly half a million extra Roth dollars compounding for the rest of your life.
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