Mega backdoor Roth: after-tax 401(k) contributions, the $72,000 limit and two ways to convert

A mega backdoor Roth uses after-tax 401(k) contributions (not Roth contributions) and turns them into Roth money, either through an in-plan Roth conversion or by withdrawing them while still employed and rolling them to a Roth IRA. For 2026 the room is the $72,000 total 401(k) limit minus your own deferrals and your employer's contributions, which can be tens of thousands of dollars. It only works if your plan allows both after-tax contributions and in-plan conversions or in-service withdrawals, and many plans don't.

How it differs from a regular backdoor Roth

A regular backdoor Roth: Happens in an IRA, is limited to $7,500 a year ($8,600 at 50+), and is subject to the IRA pro-rata rule.

A mega backdoor Roth: Happens inside the 401(k), allows far more, and isn't affected by the IRA pro-rata rule. You can do both in the same year.

Roth 401(k) contributions are something else: Roth deferrals count against the $24,500 employee deferral limit. After-tax contributions are a separate category that counts only against the $72,000 total.

How much you can put in

The formula: 2026 after-tax room = $72,000 − your deferrals (pre-tax plus Roth) − employer matching and profit-sharing contributions. Catch-ups at 50+ don't count toward the $72,000.

Example (hypothetical numbers, to show the method): At 45, with $24,500 of deferrals and a $10,000 employer match, you can add $72,000 − $24,500 − $10,000 = $37,500 of after-tax contributions in 2026. With only a $5,000 match, the room is $42,500.

Compensation limits apply too: Total annual additions can't exceed your compensation, and the plan may cap after-tax contributions at a percentage of pay.

Two ways to convert

In-plan Roth rollover: After-tax money is converted to the plan's Roth account without leaving the 401(k). Some plans can do this automatically after each contribution.

In-service distribution: While you're still employed, the after-tax portion is paid out and rolled directly to a Roth IRA.

Earnings are taxable: The after-tax contributions themselves convert tax-free, but any earnings between contribution and conversion are ordinary income when converted. That's why people convert quickly or use automatic conversions.

Splitting basis and earnings: Notice 2014-54

Under IRS Notice 2014-54, a single distribution from a 401(k) can send the pre-tax part (including earnings on after-tax money) directly to a traditional IRA and the after-tax contributions directly to a Roth IRA, without blending them pro rata. That's the basis for handling after-tax money in in-service distributions and when you leave a job.

What to confirm first

The plan document: Look in your 401(k)'s summary plan description for "after-tax contributions" and for "in-plan Roth conversion" or "in-service withdrawal." You need both.

Nondiscrimination testing (ACP): After-tax contributions, together with matching contributions, must pass a test comparing higher-paid and other employees. Where rank-and-file participation is low, highly compensated employees (prior-year pay above $160,000 for 2026) may have some after-tax contributions refunded.

Payroll pacing: After-tax contributions usually come out of payroll, so if you start mid-year you'll need a higher percentage to fill the room by December.

Cash flow: There's no deduction; it's money you've already paid tax on.

Can the self-employed do it?

The mechanics are the same in a Solo 401(k) whose plan document allows after-tax contributions and in-plan conversions, but many off-the-shelf Solo 401(k) documents don't, so ask the provider. For S corporation owners, the limits are based on W-2 wages.

When you leave the job or retire

Unconverted after-tax money still in the 401(k): When you leave, Notice 2014-54 lets you roll the after-tax contributions directly to a Roth IRA and the pre-tax part (including earnings on the after-tax money) directly to a traditional IRA, or leave it in a new employer's 401(k). Using direct rollovers for both avoids the 20% withholding.

Don't roll it all into a traditional IRA: After-tax contributions rolled into a traditional IRA become nondeductible basis there. Later withdrawals and conversions are then figured pro rata, and that basis also gets in the way of future backdoor Roth contributions.

Keep the records: Your plan's annual statements show the cumulative after-tax contributions. That number is your basis, and it comes back to you tax-free.

Three common misconceptions

"Roth 401(k) contributions are a mega backdoor." They aren't. Roth 401(k) deferrals use the $24,500 employee limit; a mega backdoor uses the after-tax room beyond it.

"If the plan allows after-tax contributions, I'm set." You also need an in-plan Roth conversion or in-service withdrawal option. Without one, the earnings keep growing as pre-tax money and are taxed when withdrawn, which is far less valuable.

"I'll let it sit and convert everything at year-end." Earnings between contribution and conversion are taxable. In a strong market, waiting months to convert can leave a meaningful amount of taxable earnings. If your plan offers automatic conversion, use it.

How it's reported

In-plan conversions and in-service distributions both produce a Form 1099-R. The after-tax contributions aren't taxed and the earnings are; box 5 of the 1099-R shows employee after-tax contributions. Once money is in a Roth IRA, the Roth IRA's own five-year clock applies.

For the full set of IRA rules, including choosing traditional or Roth, backdoor and mega backdoor Roths, conversions, early withdrawal exceptions, RMDs, inherited IRAs and cross-border status, see our guide to IRAs from first contribution to final withdrawal. To run your own numbers, book a free 15-minute call or add WeChat YCLUSA. YCL CPA doesn't sell financial products or manage investments; we get the tax right and the forms filed, and results depend on your facts and IRS rules.