Mega Backdoor Roth: After-Tax Limits, Conversion Paths, and Rules

The mega backdoor Roth is a way to move tens of thousands of dollars a year into a Roth account by making after-tax contributions to your 401(k) and then converting them to Roth. In 2026, the total defined contribution plan limit is $72,000, and after subtracting your elective deferrals and any employer match, many workers have $20,000 to $40,000 of room left for after-tax contributions that can flow into a Roth. Two conditions have to be met before any of this works: your plan must allow after-tax contributions, and it must offer a way to convert them (either in-plan or by rollover) while you’re still employed. Nondiscrimination testing can shrink the practical limit for higher earners.

What Your Plan Has to Offer

Two features have to be written into the plan document. Without both, this strategy isn’t available to you no matter how much room the IRS limits would otherwise allow.

The first is after-tax contributions. These are a separate category from pre-tax deferrals and from Roth elective deferrals under Section 402A.¹ After-tax dollars come from money you’ve already paid income tax on, but they sit in a separate sub-account where earnings grow tax-deferred rather than tax-free. Most plans don’t offer this option because most employees never ask for it. Check your Summary Plan Description or call the plan administrator directly.

The second is a way to move those after-tax dollars into a Roth account while you’re still working. That usually means the plan permits in-service distributions (so you can roll funds out to a Roth IRA) or in-plan Roth conversions (moving the money into the plan’s own designated Roth account). Since 2013, plans have been allowed to offer in-plan Roth rollovers even for amounts that wouldn’t otherwise be distributable.² Some plans go a step further and offer automatic conversion, sweeping every after-tax dollar into the Roth account each pay period without any action from you. If your plan does this, use it.

If the plan permits after-tax contributions but no conversion mechanism, your after-tax money sits in the plan earning taxable gains with no Roth path until you leave the company. That’s not fatal, but it removes most of the point.

Calculating Your After-Tax Room in 2026

Start with the total annual limit for all money going into a defined contribution plan under Section 415(c). For 2026, that ceiling is $72,000 for participants under 50.³ That cap covers everything: your elective deferrals (pre-tax or Roth), employer matching, employer profit-sharing, and after-tax contributions. Whatever’s left after the first three is your after-tax room.

In 2026, the elective deferral limit is $24,500.⁴ Say your employer matches 50% of your deferrals on a $120,000 salary, contributing $12,250. Your after-tax room is $72,000 minus $24,500 minus $12,250, or $35,250. Run the same calculation with your own numbers; employer match formulas and any profit-sharing contributions vary widely.

Catch-Ups Sit Outside the 415(c) Limit

If you’re 50 or older, the 2026 catch-up is $8,000, bringing total possible elective deferrals to $32,500.⁴ Workers who turn 60, 61, 62, or 63 during 2026 get an enhanced catch-up under SECURE 2.0 equal to the greater of $10,000 (indexed) or 150% of the standard catch-up, which works out to $12,000 for 2026.

Catch-up contributions are excluded from the 415(c) limit entirely, so they don’t eat into your after-tax room.⁵ A 52-year-old deferring $32,500 ($24,500 regular plus $8,000 catch-up) with a $10,000 employer match calculates after-tax room as $72,000 minus $24,500 minus $10,000, or $37,500. The catch-up sits outside the equation.

Nondiscrimination Testing Can Cut Your Room

The IRS doesn’t let higher earners load up on after-tax contributions while lower-paid employees contribute nothing. After-tax contributions are subject to the Actual Contribution Percentage (ACP) test, which compares contribution rates of highly compensated employees to everyone else.⁶ For 2026, you’re highly compensated if you earned more than $160,000 from the employer in the prior year or owned more than 5% of the business at any point.³

The ACP test caps highly compensated employees at either 125% of the non-highly-compensated group’s average contribution percentage, or that average plus two points (limited to 200% of their average), whichever is greater.⁶ If few non-HCEs make after-tax or matched contributions, your practical after-tax room might be a small fraction of the $72,000 theoretical cap.

When a plan fails the test, the employer must refund excess contributions to the affected employees. Those refunds are taxable in the year of distribution, can’t be rolled to a Roth, and trigger a 10% excise tax on the employer if they aren’t returned within two and a half months after the plan year ends.⁶ This is where the strategy stumbles at big companies. You think you have $35,000 of room, contribute accordingly, and get a check back in March with a tax bill.

Solo 401(k) Plans Avoid the Problem

Self-employed workers with a solo 401(k) have a real advantage. Because the plan covers only the business owner and possibly a spouse, there are no non-highly-compensated employees to compare against, and no ACP testing applies. If your solo 401(k) plan document allows after-tax contributions and in-plan Roth conversions, you can use the full after-tax room without worrying about refunds.

The Two Conversion Paths

Once after-tax dollars are in the plan, you need to move them into a Roth account. Two options exist, and the choice affects where the money ends up.

In-Plan Roth Conversion

This keeps the money inside your employer’s plan, moving it from the after-tax sub-account to a designated Roth account within the same 401(k). The entire amount converts at once, including any earnings. You owe income tax on the earnings portion in the year of conversion. The contributed dollars themselves aren’t taxed again because you already paid income tax on them.

Rollover to a Roth IRA

This moves the money out of the 401(k) and into a Roth IRA at a brokerage. Under IRS Notice 2014-54, when you take a distribution containing both after-tax contributions and pre-tax earnings, you can split it: send the after-tax basis to a Roth IRA and the pre-tax earnings to a traditional IRA.⁷ Doing it this way converts only the already-taxed dollars into the Roth IRA and defers tax on the earnings in a traditional IRA. That’s a cleaner result than an in-plan conversion when earnings have piled up.

One constraint applies. You can’t take a distribution of only the after-tax dollars while leaving everything else in the plan. Any partial distribution has to include a proportional share of pre-tax and after-tax amounts.⁷ The Notice 2014-54 split-rollover technique handles this by sending each portion to a different destination at the same time.

Convert Quickly to Keep the Tax Bill Small

Any investment earnings on your after-tax contributions between the contribution date and the conversion date are taxable income in the year you convert. The contributed dollars owe no additional tax; the gains are treated as pre-tax.⁷ Contribute $10,000 and convert the next day, and the earnings are negligible. Wait six months during a rising market and you might owe tax on hundreds or thousands of dollars in gains.

The clean approach is to convert after every payroll contribution, weekly or biweekly. Plans with automatic in-plan Roth conversion do this for you. Without that feature, set a calendar reminder to initiate the conversion each pay period. This one habit separates a nearly tax-free mega backdoor Roth from one that produces a surprise bill each April.

Requesting the Conversion

Process varies by administrator. Most large custodians offer an online portal for initiating an in-service distribution or in-plan conversion. Some require a call to a plan representative on a recorded line. A smaller number still use paper forms with names like “In-Service Distribution Request” or “Roth Conversion Election Form.”

On the request, specify that you’re moving only the after-tax portion, identify the destination (the plan’s Roth account for an in-plan conversion, or an external Roth IRA with full account details for a rollover), and confirm the transfer is a direct rollover. Always pick a direct rollover. An indirect rollover gives you 60 days to redeposit and triggers mandatory 20% federal withholding on the pre-tax portion, which creates a cash-flow problem you don’t need.

How the Conversion Gets Reported

After year-end, the plan custodian issues Form 1099-R for the conversion. Box 1 shows the total distributed, Box 2a shows the taxable portion (usually just the earnings), and Box 5 shows the after-tax basis recovered.⁸ Box 7 shows distribution code G, which the IRS uses for both direct rollovers to a Roth IRA and in-plan Roth conversions.⁹

On Form 1040, the total from Box 1 goes on the pensions and annuities line (5a), and the taxable amount from Box 2a goes on line 5b. If you converted right after contributing and the taxable amount is near zero, line 5b will show $0 while line 5a shows the full converted amount. That mismatch is normal.

Form 8606 tracks the basis of amounts you’ve converted or rolled into Roth IRAs. Line 24 accounts for prior conversions and rollovers from qualified plans, which determines how much of a future distribution is tax-free.¹⁰ Keep every 1099-R. If the IRS questions a distribution years later, those forms are how you prove the money was already taxed.

The Five-Year Rule on Converted Amounts

Money converted to a Roth IRA through this strategy is subject to a five-year holding period before converted amounts can come out penalty-free if you’re under 59½. Each conversion starts its own five-year clock, beginning January 1 of the year of the conversion. Withdraw converted amounts before that period ends and before reaching 59½, and you face a 10% early withdrawal penalty on the portion that was taxable at conversion (the earnings). After 59½, the penalty doesn’t apply regardless of holding period.

The after-tax basis portion of each conversion (the money you already paid tax on) comes out first under Roth IRA ordering rules and is never taxed or penalized again. The clock really only matters for the earnings that were included in the conversion, which is another reason to convert quickly and keep that taxable slice small.

For in-plan Roth conversions that stay inside the 401(k), the five-year rule works differently. The plan’s own distribution restrictions govern access to those funds, which generally means no withdrawal until a distributable event occurs (separation from service, age 59½, disability, or plan termination).

SECURE 2.0 Provisions That Interact With This Strategy

Two SECURE 2.0 changes are worth understanding.

Mandatory Roth Catch-Ups for High Earners

For taxable years beginning after December 31, 2025, employees whose wages from the plan sponsor exceeded $150,000 in the prior year must make any catch-up contributions as Roth deferrals rather than pre-tax.¹¹ This doesn’t change the mega backdoor math, since catch-ups sit outside 415(c) either way, but it does require the plan to have a designated Roth account. If a plan lacks one, affected employees can’t make catch-up contributions at all, which may prompt plan amendments that open the door to after-tax features as well.

Employer Roth Matching

Plans can now allow employer matching and nonelective contributions to be designated as Roth.¹² If your employer offers this and you elect Roth for the match, those dollars still count toward the 415(c) limit exactly as pre-tax matches do. The after-tax room calculation is the same either way.

Legislative Risk

The mega backdoor Roth has faced repeated elimination attempts in Congress. Several recent proposals would have prohibited after-tax contributions from being converted to Roth accounts, effectively ending the strategy. None had become law as of 2026. If you have both the plan features and the contribution room, financial planners generally recommend using the strategy while it remains available rather than counting on it being there indefinitely.

1
Office of the Law Revision Counsel. 26 USC 402A – Optional Treatment of Elective Deferrals as Roth Contributions
2
Internal Revenue Service. Deadline Extended to Add New In-Plan Roth Rollover Provisions
3
Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
4
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
5
Internal Revenue Service. Application of IRC Section 415(c) When a 403(b) Plan Is Aggregated With a Section 401(a) Defined Contribution Plan
6
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
7
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
8
Internal Revenue Service. Instructions for Forms 1099-R and 5498
9
Internal Revenue Service. Instructions for Forms 1099-R and 5498 (PDF)
10
Internal Revenue Service. Instructions for Form 8606
11
Internal Revenue Service. Treasury, IRS Issue Final Regulations on New Roth Catch-Up Rule, Other SECURE 2.0 Act Provisions
12
Internal Revenue Service. SECURE 2.0 Act Changes Affect How Businesses Complete Forms W-2