How the mega backdoor Roth works
A 401(k) can hold three kinds of employee money: pre-tax deferrals, Roth deferrals and after-tax contributions. The first two share the elective-deferral limit, $24,500 for 2026. After-tax contributions aren’t elective deferrals, so they don’t touch that limit. They count only against the plan’s overall ceiling on annual additions under section 415(c): $72,000 for 2026, or 100% of your pay if that is less.
On their own, after-tax contributions are a middling deal. You pay tax on the money before it goes in, and its growth is taxed as ordinary income when it comes out. The mega backdoor fixes that by moving the money into Roth soon after it arrives, through one of two routes. Either way, the after-tax dollars move untaxed, because they were already taxed as pay. Earnings on them are pre-tax money, so whatever growth you convert is income that year. The more often your plan lets you convert, the smaller that taxable slice.
- An in-plan Roth rollover moves after-tax money to the plan’s own Roth 401(k) account. Since 2013 plans may offer this even for money you couldn’t otherwise withdraw.
- An in-service withdrawal pays after-tax money out while you still work, rolled directly to a Roth IRA; any earnings in it are taxed or sent to a traditional IRA.
How much can you put in a mega backdoor Roth in 2026?
Your room is whatever the $72,000 annual additions limit has left after your own deferrals and everything your employer puts in: matching contributions, profit-sharing contributions and any forfeitures reallocated to you. Catch-up contributions sit outside the $72,000, so they neither add to nor use up after-tax room. With catch-ups, total additions can reach $80,000 at 50 or older, or $83,250 in the years you turn 60 through 63.
Take someone under 50 who defers the full $24,500 and receives a $10,000 employer match. That leaves $72,000 − $24,500 − $10,000 = $37,500 of after-tax room. Because employer money counts too, your room isn’t final until the employer’s contributions for the year are known.
The annual additions limit applies per employer, across all plans that employer and any related employer maintain, so a second job with an unrelated employer brings a separate $72,000. Your $24,500 of elective deferrals, by contrast, is shared across every plan you’re in. Your plan document can also set a lower cap on after-tax contributions than the law allows.
How to set up a mega backdoor Roth
Most of the mega backdoor is plan design rather than tax law. It can’t be done through an IRA, and your employer decides whether the features exist; many plans have none of them. Without a way to convert, after-tax money grows with its earnings taxed at ordinary rates on the way out, which can end up worse than a taxable brokerage account, where long-held gains get capital-gains rates. Start with the summary plan description or the plan administrator, then work through these steps:
- Confirm the plan accepts after-tax employee contributions that aren’t Roth, and note any cap as a share of pay.
- Find out whether it offers in-plan Roth rollovers of after-tax money, or in-service withdrawals of it, and how often you can make them.
- Set your pre-tax or Roth deferrals, estimate the employer’s contributions, then elect an after-tax rate that fits the room left.
- Convert or withdraw soon after each contribution, so little taxable growth builds up.
- Ask whether highly compensated employees have had after-tax money refunded after a failed nondiscrimination test.
Nondiscrimination testing and other catches
After-tax contributions count in the actual contribution percentage (ACP) test, the yearly check that highly compensated employees aren’t contributing far more, as a share of pay, than everyone else. For 2026 testing you are generally highly compensated if you were paid more than $160,000 by the employer in the prior year or own more than 5% of the business. When the test fails, the plan must correct it, typically by returning the excess to the high earners.
A safe-harbor 401(k) doesn’t make the problem go away. The Treasury regulation on the ACP safe harbor covers matching contributions only, so after-tax employee contributions still have to pass the test. That is why real-world mega backdoor room can be smaller than the $72,000 arithmetic suggests, and why it can change from year to year.
Two smaller catches. An in-plan Roth rollover can’t be undone once made. And the IRS doesn’t require withholding on a direct in-plan rollover, so if earnings have built up you may need higher withholding or estimated tax payments to cover the tax.
Mega backdoor Roth vs. the backdoor Roth IRA
The two share a name and a goal but little else. The backdoor Roth IRA runs through a traditional IRA, needs no employer, and moves a small fixed amount each year. Its main hazard is the pro-rata rule, which pre-tax money in any of your IRAs triggers. The mega backdoor runs inside a workplace plan, can move several times as much, and its main hazards are plan design and testing.
Because IRA limits and workplace-plan limits are separate, the two stack. A 401(k) balance doesn’t enter the IRA pro-rata calculation, so converting inside the plan leaves the IRA version clean. An in-service withdrawal is different if you send its earnings to a traditional IRA: that pre-tax IRA money counts on December 31. For one worker under 50 with a $10,000 match, $24,500 of Roth deferrals, $37,500 of converted after-tax money and a $7,500 backdoor add up to $69,500 of Roth savings in 2026.
Illustrative numbers
A single 40-year-old earning $200,000 with a 5% match in 2026
- $72,000
- The 2026 section 415(c) annual additions limit, or 100% of pay if lower
- Elective deferrals
- Your pre-tax and Roth 401(k) deferrals for the year
- Employer contributions
- Matching and profit-sharing contributions for the year
- Forfeitures
- Forfeited balances reallocated to your account
Your plan can set a lower cap, and nondiscrimination testing can cut the result for highly compensated employees.
Annual additions limit, under 50$72,000
Roth 401(k) deferrals−$24,500
Employer match, 5% of pay−$10,000
After-tax room, converted in-plan$37,500
Earnings before conversions, assumed$150
Tax on those earnings at a 24% bracket$36
She can add $37,500 of after-tax money and convert it, putting $62,000 into Roth for the year counting her Roth deferrals, for $36 of tax. If the plan fails its ACP test, part of the $37,500 may come back to her. Left unconverted for decades, every dollar of growth on it would have been ordinary income instead of tax-free once she meets the five-year and age tests.
At a glance
Backdoor Roth IRA vs. mega backdoor Roth, 2026
| Feature | Backdoor Roth IRA | Mega backdoor Roth |
|---|---|---|
| Money goes through | A nondeductible traditional IRA | After-tax contributions to a workplace plan |
| 2026 amount | Up to $7,500 ($8,600 at 50+) | Up to $72,000, less deferrals and employer money |
| Income limit | None | None, but ACP testing can cap high earners |
| Needs employer support | No | Yes: after-tax contributions plus in-plan conversion or in-service withdrawal |
| Roth destination | Roth IRA | Roth 401(k) or Roth IRA |
| Main tax trap | Pro-rata rule on other pre-tax IRA money | Tax on earnings when conversion is delayed |
Put it in your plan
Mega backdoor Roth in MoneyWhatIf
MoneyWhatIf applies the 2026 total-additions ceiling to each matched account: $72,000 for employee and employer money together under 50, rising with catch-ups to $80,000 at 50 or older and $83,250 at 60 to 63. An employer match is capped by whatever room that ceiling has left, and after-tax requests are scaled back when take-home pay can’t cover them. Nondiscrimination testing isn’t fully represented, and the model can’t confirm that your plan permits the conversions or rollovers this strategy needs.
Common questions
Mega backdoor Roth FAQs
Is it better to convert inside the plan or roll to a Roth IRA?
It depends on what your plan offers and when you might need the money. Rolled to a Roth IRA, after-tax dollars count as rollover contributions, and the Roth IRA ordering rules take them out before any earnings, free of income tax and the 10% additional tax. A nonqualified withdrawal from a Roth 401(k) is instead a proportional mix of contributions and earnings, and the plan runs its own five-year clock. Since 2024, neither account requires lifetime minimum distributions.
Do I pay tax when I convert after-tax contributions?
Only on the earnings. The contributions came from pay you were already taxed on, so they are basis. An in-plan Roth rollover adds its taxable amount, the value minus your basis, to that year’s income. With an in-service withdrawal, you can send the earnings to a traditional IRA instead and owe nothing now, though that IRA money then makes a backdoor Roth IRA partly taxable. Converting soon after each contribution keeps the taxable part small.
Does the mega backdoor Roth have an income limit?
No law limits income for after-tax contributions or Roth conversions. The practical limit is your plan’s ACP test, which compares the matching and after-tax contributions of highly compensated employees with everyone else’s. When rank-and-file employees contribute little, the plan may cap high earners’ after-tax contributions in advance or refund part of them after the year ends.
What if my plan allows after-tax contributions but no conversions?
The benefit shrinks. The money grows tax-deferred, but its earnings are taxed as ordinary income when withdrawn, and only the contributions come back tax-free. You can still clean it up when you leave the job: a full distribution can be split so the after-tax contributions go to a Roth IRA and the earnings, with other pre-tax money, to a traditional IRA, with no tax due.
Is converted money subject to a five-year rule?
Yes, in two ways. If an in-plan rollover is the first Roth money in your plan, it starts that plan’s five-year clock for qualified withdrawals. And withdrawing the taxable part of an in-plan rollover within five years, before 59½, can trigger the 10% additional tax. Converted after-tax contributions were never taxable, so that recapture doesn’t reach them. The Roth five-year rules page covers every clock.