A mega backdoor Roth lets you contribute far more than the $24,500 401(k) elective deferral limit into Roth accounts each year, up to the $72,000 total annual additions cap for 2026. It works in two steps: make after-tax (non-Roth) contributions to your 401(k), then convert them to Roth. Your plan must allow both steps, which is the part most people miss.

How does a mega backdoor Roth work?

The standard 401(k) elective deferral limit for 2026 is $24,500. But a separate, higher limit caps everything that goes into your 401(k) in a year: employee deferrals, employer match, profit sharing, and after-tax contributions combined. For 2026, that total annual additions limit under Section 415(c) is $72,000. (IRS Notice 2025-67)

The mega backdoor Roth fills the gap between those two numbers with after-tax contributions, then converts them to Roth. The sequence:

1. Contribute after-tax dollars to your 401(k) beyond your $24,500 of pre-tax or Roth elective deferrals, up to the $72,000 total cap (minus employer contributions).

2. Convert the after-tax balance to Roth, either through an in-plan Roth rollover into the plan's designated Roth account or by rolling it out to a Roth IRA.

3. Repeat each year. The headroom resets annually.

Illustrative example: you max the $24,500 elective deferral and your employer contributes a $10,000 match. Total additions are $34,500, leaving $37,500 of headroom under the $72,000 cap. If your plan permits it, that $37,500 goes in after-tax and converts to Roth.

What is the difference between after-tax and Roth 401(k) contributions?

People confuse these constantly. They are different buckets with different tax treatment.

After-tax contributions are the raw material of the mega backdoor. On their own they are a mediocre deal, because their earnings grow tax-deferred, not tax-free. The conversion step is what turns them into Roth dollars.

Which 2026 limits matter?

Three numbers control the size of your mega backdoor:

• $24,500: the elective deferral limit (pre-tax plus Roth 401(k) combined). After-tax contributions do not count against this. (IRS IR-2025-111)

• $72,000: the Section 415(c) total annual additions limit per participant. Everything counts against this. (IRS Notice 2025-67)

• Your employer contributions: match and profit sharing eat into the $72,000 cap first. Your after-tax headroom is $72,000 (or 100% of your compensation, if less) minus your regular elective deferrals (not counting catch-up contributions) minus employer contributions and any forfeitures allocated to you.

If you are 50 or older, catch-up contributions ($8,000 for 2026, or $11,250 if you turn 60, 61, 62 or 63 during the year) do not count toward the $72,000 cap, so your total 401(k) additions can reach $80,000 (or $83,250 at ages 60-63). One 2026 change to note: under SECURE 2.0, if your 2025 FICA wages from the employer sponsoring the plan exceeded $150,000, your 2026 catch-up contributions must be designated Roth. If the plan has no Roth option, you cannot make catch-up contributions at all. (IRS Notice 2025-67)

What must your 401(k) plan allow?

Two features, and you need both. There is also a third constraint that often matters most for high earners: after-tax contributions, together with employer matching contributions, must pass the ACP nondiscrimination test under IRC 401(m). If you are a highly compensated employee (for 2026, generally 2025 compensation above $160,000), the plan may cap your after-tax contributions or refund part of them after year-end. Ask your administrator whether HCE after-tax contributions were limited or refunded in recent years.

1. After-tax contributions. Check your plan's summary plan description or contribution election screen for a contribution type labeled "after-tax" (distinct from "Roth"). Many large-employer plans offer it; many small-business plans do not.

2. A conversion path while you are still employed. Either in-plan Roth conversions (converting inside the 401(k) to the designated Roth account) or in-service distributions/withdrawals (rolling the after-tax money out to a Roth IRA while still working there).

If your plan allows after-tax contributions but no conversion path, the money sits there growing tax-deferred with taxable earnings, which defeats the purpose. Ask your plan administrator directly: "Does the plan allow after-tax contributions, and does it allow in-plan Roth conversions or in-service distributions of after-tax amounts?" Get the answer in writing.

How do you execute the conversion?

Two routes, both blessed by IRS guidance:

• In-plan Roth rollover. Convert the after-tax balance to the plan's designated Roth account. The IRS confirmed that plans may allow these rollovers even for amounts that are not otherwise distributable, and a direct in-plan rollover has no withholding. (IRS Notice 2013-74) The converted amount keeps whatever distribution restrictions applied to it before the rollover (Notice 2013-74, Q&A-3). For money that was not otherwise distributable, that usually means no withdrawal before 59½ while you are employed. After-tax contributions are often withdrawable in service under the plan's terms, so check your plan document.

• Split rollover to IRAs. Take a distribution and send the after-tax portion to a Roth IRA and the pre-tax earnings to a traditional IRA. Notice 2014-54 explicitly permits allocating the distribution this way so the Roth-bound dollars consist entirely of after-tax amounts. (IRS Notice 2014-54)

What gets taxed at conversion? Only the earnings. Your after-tax contributions are already-taxed principal, so converting them is tax-free; any growth that accumulated before conversion is taxable as ordinary income in the conversion year. That is why frequent converters automate the process: convert quickly and there is barely any taxable earnings.

What are the common mistakes?

• Assuming every 401(k) allows it. Most do not. The plan document controls, and smaller plans frequently omit after-tax contributions.

• Letting earnings accumulate before converting. Every month the after-tax money sits unconverted, taxable earnings grow. Convert promptly, ideally automatically each pay period if the plan supports it.

• Forgetting employer contributions count toward $72,000. A generous profit-sharing contribution shrinks your after-tax headroom dollar for dollar.

• Mixing it up with the regular backdoor Roth IRA. They are separate strategies with separate limits ($7,500 IRA vs. $72,000 415(c)). You can do both in the same year.

Pending review by Brian Thomas, EA. Updated September 25, 2026.

This article is free educational content, not tax advice for your specific situation. If you want a plan built around your actual numbers, that is what our paid tax planning engagements do.