What is a Mega Backdoor Roth?

A Mega Backdoor Roth is a strategy that moves after-tax contributions from a 401(k) into a Roth account, letting you put far more into Roth savings than the standard Roth IRA or Roth 401(k) limits allow on their own. You make extra after-tax contributions to your workplace plan, then convert that money to Roth so it can grow tax-free.

The “mega” distinguishes it from the ordinary backdoor Roth IRA, which moves at most a few thousand dollars a year through a nondeductible IRA. The mega version runs through your 401(k) instead, where the total contribution ceiling is much higher, so a saver who has already maxed out every other tax-advantaged account can direct tens of thousands of additional dollars into Roth annually.

How the Mega Backdoor Roth Works

The strategy relies on the gap between what you can contribute to a 401(k) as an employee and the higher overall limit the Internal Revenue Code (IRC) sets on total contributions to the plan. It comes together in three steps.

1. Contribute your elective deferral. Put in up to the standard employee limit through pre-tax or Roth 401(k) contributions. For 2026 that limit is $24,500, or $32,500 if you are 50 or older.
2. Add after-tax contributions. If your plan allows it, contribute additional after-tax dollars toward the overall Section 415© limit on everything that can go into the plan. For 2026 that combined limit is $72,000, or $80,000 for those 50 and older, and it counts your elective deferral, any employer match, and your after-tax contributions together.
3. Convert the after-tax money to Roth. Move those after-tax contributions into Roth through an in-plan Roth conversion or a rollover to a Roth IRA. This is the step that turns ordinary after-tax savings into a Roth balance that grows and comes out tax-free.

Does My 401(k) Allow a Mega Backdoor Roth?

The strategy only works if your employer’s plan supports two specific features, and many plans support neither:

• After-tax contributions. These are a separate contribution type, distinct from pre-tax and Roth 401(k) deferrals. Without an after-tax bucket, there is no extra money to convert.
• In-service conversions or withdrawals. You need a way to move the after-tax money into Roth while you are still employed, either through an in-plan Roth conversion or an in-service withdrawal you roll to a Roth IRA. If the plan makes you wait until you leave, the strategy stalls.

Confirm both with your plan administrator or in the summary plan description before you start. Solo 401(k) plans for the self-employed can also support the strategy, but only if the plan document specifically allows after-tax contributions and in-plan conversions, which many off-the-shelf providers do not.

The Pro-Rata and Earnings Nuance

Timing matters because of what happens to investment earnings. After-tax contributions themselves convert to Roth tax-free, since you already paid tax on that money. But any earnings those contributions generate before you convert are pre-tax, so converting them adds to your taxable income for the year.

Convert soon after each contribution and there is little or no growth to be taxed. Let the after-tax money sit and compound for months, and the gains you eventually convert become a taxable event. Some plans handle this automatically by sweeping after-tax contributions into Roth on a set schedule; others leave the timing to you. If earnings do accumulate, plans generally apply them pro-rata, so a conversion pulls a proportional slice of taxable earnings along with your tax-free contributions rather than letting you cherry-pick only the contributions.

Mega Backdoor Roth Limit in 2026: A Worked Example

Say you are under 50 and your employer offers no matching contribution. You first contribute the full $24,500 elective deferral. The overall 2026 limit on total contributions to the plan is $72,000, which leaves $72,000 - $24,500 = $47,500 of room for after-tax contributions.

You contribute that $47,500 in after-tax dollars, then convert the full amount to a Roth IRA or Roth 401(k) through an in-plan conversion or rollover. On top of the $24,500 already in your 401(k), you have moved $47,500 into Roth in a single year, well beyond the $7,500 that direct Roth IRA contributions would have allowed.

An employer match changes the arithmetic. Because the match counts toward the same $72,000 ceiling, a $10,000 employer contribution would shrink your after-tax room to $37,500. Work out your own number by subtracting both your elective deferral and any expected employer contributions from the overall limit.

If you want to see how a Mega Backdoor Roth reshapes your future tax picture, you can model the conversions in ProjectionLab alongside your other accounts and compare the long-term outcome against skipping the strategy.

Frequently Asked Questions

How much can you put in a mega backdoor Roth in 2026? Up to $47,500 if you are under 50, contribute the full $24,500 elective deferral, and receive no employer match. That figure is the $72,000 overall 2026 limit minus your $24,500 deferral. Any employer contributions reduce the after-tax room dollar for dollar, and savers 50 and older work against the higher $80,000 overall limit.

What is the difference between a mega backdoor Roth and a backdoor Roth? Scale and mechanics. A backdoor Roth IRA moves up to the annual IRA limit ($7,500 in 2026) through a nondeductible traditional IRA, and it exists to get around the Roth IRA income phase-outs (which run from $153,000 to $168,000 for single filers and $242,000 to $252,000 for married couples filing jointly in 2026). A Mega Backdoor Roth runs through your 401(k) instead, using after-tax contributions and in-plan conversions to move as much as $47,500 or more. Many high earners use both in the same year.

What are the tax implications of a mega backdoor Roth? The after-tax contributions convert to Roth tax-free because you already paid income tax on them. Only the investment earnings that accrue between contributing and converting are taxable, which is why converting quickly keeps the tax bill near zero. Once inside the Roth account, the money grows and, in qualified distributions, comes out entirely tax-free.