Mega Backdoor Roth: How It Works and the 2026 Limit

A mega backdoor Roth puts after-tax (non-Roth) 401(k) contributions into your plan and then moves them into Roth status, either inside the plan or by rolling them to a Roth IRA. For 2026 the ceiling is the IRS total plan limit of $72,000, minus your own $24,500 of deferrals and whatever your employer puts in. With no employer money that leaves $47,500 of room, more than six times the $7,500 IRA limit. It has no income limit, but it only works if your plan offers the right features. Here is the math, the plan checklist, the tax rules and the steps.

The Short Answer

• What it is: after-tax employee contributions to a 401(k) (or 403(b)) that you convert to Roth, through an in-plan Roth conversion or an in-service rollover to a Roth IRA.
• 2026 room: $72,000 minus your elective deferrals minus employer contributions. Catch-up contributions ($8,000 at 50+, $11,250 at ages 60 to 63) sit on top and do not use up the room.
• Income limit: none. That is the point for high earners shut out of direct Roth IRA contributions.
• The catch: your plan must allow after-tax contributions and a way to get them into Roth. Not every plan does.
• The tax: the after-tax contributions convert tax-free. Earnings on them before you convert are taxable, so convert quickly.
• The test: if you are a highly compensated employee (2025 pay over $160,000 from that employer, or a 5% owner), the ACP nondiscrimination test can cap or refund your after-tax money.

How the Mega Backdoor Roth Works

A 401(k) can hold three kinds of employee money: pre-tax deferrals, Roth deferrals, and a less common third kind, after-tax (non-Roth) contributions. The first two share the $24,500 elective deferral limit for 2026. After-tax contributions do not count toward that limit. They count only toward the larger section 415(c) "annual additions" limit, which the tax code defines as the sum of employer contributions, employee contributions and forfeitures.

On their own, after-tax contributions are a weak deal: you get no deduction, and the earnings are taxed as ordinary income when withdrawn. The mega backdoor fixes that by moving the money into a Roth account soon after it goes in. From then on, growth is tax-free if you take a qualified distribution later. Two routes do this:

• In-plan Roth conversion (the IRS calls it an in-plan Roth rollover): the plan moves the after-tax money into your designated Roth account in the same 401(k). The plan must have a Roth 401(k) option for this to exist.
• In-service withdrawal to a Roth IRA: while still employed, you take the after-tax money out and roll it directly to a Roth IRA. Under IRS Notice 2014-54 the after-tax dollars go to the Roth IRA and the pre-tax earnings can go to a traditional IRA in the same distribution.

The 2026 Limit and How Much Room You Have

These are the 2026 figures from IRS Notice 2025-67:

Source: IRS Notice 2025-67; IRS 401(k) contribution limits page; 26 U.S.C. 414(v)(3) for the catch-up exclusion. The cap is also limited to 100% of your compensation, and it applies per employer.

So the formula is: after-tax room = $72,000 − your elective deferrals − employer contributions, and never more than your pay allows.

Hypothetical example. Priya, 45, earns $180,000 and defers the full $24,500. Her employer matches 50% of the first 6% of pay, which is $5,400. Her after-tax room is $72,000 − $24,500 − $5,400 = $42,100.

Hypothetical, our arithmetic. The last row hits the 100%-of-compensation cap: $60,000 − $26,300 = $33,700, before taxes and living costs make that amount unrealistic.

Two details matter. First, the catch-up does not shrink the after-tax room, because catch-ups are outside the 415(c) limit. If your 2025 wages from that employer were over $150,000, your 2026 catch-up must go in as Roth. Second, employer money counts in full, so a year-end true-up or profit-sharing deposit can use room you thought you had. Our 2026 401(k) limits page covers the deferral and catch-up rules in detail.

What Your Plan Must Allow

How to check:

• Read the summary plan description (SPD). Look for "after-tax contributions" or "voluntary after-tax", and for "in-plan Roth rollover" or "in-service withdrawal". A Roth 401(k) option alone is not the same thing.
• Ask the plan administrator three questions: Is there a cap on after-tax contributions as a percentage of pay? Can I convert in-plan, and how often? Does the plan track after-tax money in its own subaccount?
• Look for an automatic conversion option. Some plans can convert each after-tax contribution as it arrives. That keeps taxable earnings near zero.

The last question matters for the tax math below. The tax code lets a plan treat employee contributions and their earnings as a separate contract (26 U.S.C. 72(d)(2)), which changes what a partial withdrawal is made of.

How to Do a Mega Backdoor Roth, Step by Step

1. Confirm the plan features above, in writing if you can.
2. Max your elective deferrals first ($24,500 for 2026, pre-tax or Roth). Deferrals get pre-tax or Roth treatment right away, they are often what your match is based on, and they come out of the same $72,000.
3. Estimate employer money for the full year, including any match true-up or profit sharing, and subtract it with your deferrals from $72,000.
4. Set your after-tax contribution rate so the year's total stays under the room you calculated and under any plan percentage cap. Spread it over the year if your match is paid per paycheck, so you do not reach the $72,000 cap early and leave no room for later match deposits.
5. Convert promptly. Turn on automatic in-plan conversion if offered, or request a conversion or in-service rollover after each contribution or on a regular schedule.
6. Split earnings correctly on a rollover. Send the after-tax contributions to the Roth IRA and the pre-tax earnings to a traditional IRA (or pay tax on them). Ask for direct rollovers so nothing is withheld.
7. Plan for the tax on earnings. An in-plan conversion may have no tax withheld (the IRS says none can be withheld when the money was not otherwise distributable), so cover any tax through paycheck withholding or estimated payments.
8. Keep records of contributions, conversions and the plan's tax forms. An in-plan conversion cannot be undone.

Taxes: Earnings, Pro-Rata and Notice 2014-54

The contributions are tax-free to convert. The earnings are not. You already paid tax on after-tax contributions, so moving them to Roth costs nothing. The IRS treats the earnings on them as pre-tax money, and an in-plan conversion adds any amount that would have been taxable to your income for the year.

Hypothetical example. If $20,000 of after-tax money sits for a year and earns an assumed 7%, the $1,400 of earnings is taxable when you convert. At an assumed 24% marginal rate that is about $336. Converting each paycheck's contribution within days keeps the earnings, and the tax, close to zero.

Pro-rata inside the plan. The IRS says you cannot take out only after-tax money and leave the rest: "Any partial distribution from the plan must include some of the pretax amounts." Each distribution carries a proportional share of pre-tax and after-tax amounts in the account. If your plan tracks after-tax money and its earnings separately, that share is figured on the after-tax subaccount, not your whole balance, which is why the plan's accounting matters.

Notice 2014-54 splits the destinations. When one distribution goes to several places at once, the IRS treats it as a single distribution and lets you send the pre-tax part to a traditional IRA (or another plan) and the after-tax part to a Roth IRA. The IRS example: from $100,000 made up of $80,000 pre-tax and $20,000 after-tax, you can roll $80,000 to a traditional IRA and $20,000 to a Roth IRA, and the Roth IRA receives only after-tax dollars.

The IRA pro-rata rule is a different animal. In a regular backdoor Roth IRA, Form 8606 mixes your conversion with every traditional, SEP and SIMPLE IRA balance you hold on December 31. A rollover out of a 401(k) is allocated at the plan level instead, so pre-tax money sitting in your own traditional IRAs does not make a mega backdoor taxable. The one link: if you send the pre-tax earnings to a traditional IRA, that IRA balance will count against any future backdoor Roth IRA conversion.

ACP Testing and Highly Compensated Employees

After-tax contributions go through the actual contribution percentage (ACP) test under section 401(m), together with matching contributions. Each employee's ACP is matching plus after-tax contributions divided by pay. The plan passes if the average for highly compensated employees (HCEs) does not exceed the greater of 125% of the non-HCE average, or the lesser of 200% of the non-HCE average and the non-HCE average plus 2 percentage points.

For 2026, you are generally an HCE if you were a 5% owner in 2025 or 2026, or if the employer paid you more than $160,000 in 2025 (the employer may also limit this group to the top-paid 20%). Because higher earners are the ones most likely to make after-tax contributions, a plan may cap them for HCEs to pass. If a plan fails, the excess can be refunded to the HCE by the end of the following plan year, with its earnings, which undoes part of your mega backdoor.

Safe harbor plans are not an escape. IRS regulations say a safe harbor plan that allows employee contributions "must also satisfy the ACP test" for them. Ask HR whether HCEs have a separate after-tax limit or have received refunds in past years.

The Five-Year Rules

• Rollover to a Roth IRA (penalty clock). Each conversion or rollover from a plan to a Roth IRA starts its own five-year period on January 1 of that year. If you are under 59½ and withdraw within that period, the 10% additional tax applies only to the part that was taxable when it went in. In a mega backdoor that is the earnings, often a few dollars, because the after-tax contributions were not taxable.
• In-plan conversion (same idea, inside the 401(k)). IRS Notice 2010-84 applies a matching five-year recapture rule to the taxable amount of an in-plan Roth rollover. The money also stays subject to the plan's own withdrawal restrictions.
• Qualified-distribution clock (earnings). Roth IRA earnings are tax-free only after five years from January 1 of the first year you funded any Roth IRA, and after age 59½, disability, death or a first-home purchase. A Roth 401(k) runs its own five-year clock; if an in-plan conversion is your first Roth 401(k) money, that clock starts January 1 of the conversion year.

Our Roth 401(k) vs Roth IRA guide compares the two Roth homes, including where you might roll the money later.

Mega Backdoor vs Backdoor Roth IRA

Sources: IRS Notice 2025-67, Notice 2014-54, Form 8606 instructions, 26 U.S.C. 401(m).

You can do both in the same year: the IRA and 401(k) limits are separate. A couple where both spouses have plan access and IRAs could, in principle, run four Roth channels. See our backdoor Roth IRA guide for the IRA side and our 2026 Roth IRA limits for the income phase-outs that make it necessary.

Solo 401(k), the TSP and the Law

Solo 401(k). The IRS says a one-participant 401(k) has "the same rules and requirements as any other 401(k) plan", and a business owner with no common-law employees does not run nondiscrimination testing. So a self-employed person can do a mega backdoor if the plan document allows after-tax contributions and either in-plan conversions or in-service distributions. Not every provider's document does, so check before you open one. The 415(c) cap uses earned income as compensation, which limits the room for lower-profit businesses. Hiring eligible employees brings testing back.

Federal TSP. The Thrift Savings Plan offers two tax treatments for employee contributions, traditional (pre-tax) and Roth (after-tax), and no separate after-tax (non-Roth) source, so there is no mega backdoor in the TSP. The TSP does offer Roth in-plan conversions of your traditional balance with no income limit, but that converts pre-tax money, so the full amount is taxable and nothing is withheld. Our TSP vs 401(k) vs IRA comparison covers the other differences.

The law. The House-passed Build Back Better Act (H.R. 5376, November 19, 2021) included section 138311, which would have barred rollovers and conversions of after-tax amounts to Roth accounts. The version signed into law in August 2022 (Public Law 117-169) did not include it, and the 2025 One Big Beautiful Bill Act (Public Law 119-21) did not change the Roth rollover rules either. The IRS page on after-tax rollovers, last updated February 2026, still describes after-tax-to-Roth-IRA rollovers as allowed.

To see what steady Roth contributions might grow to, try our 401(k) calculator.

Sources & Methodology

• IRS Notice 2025-67, 2026 section 415(c) limit, elective deferral limit, catch-up limits, Roth catch-up wage threshold and HCE threshold; Notice 2024-80 for the 2025 HCE threshold.
• IRS, 401(k) and profit-sharing plan contribution limits, for the 100%-of-compensation rule and the $80,000 and $83,250 totals.
• IRS, Rollovers of after-tax contributions and Notice 2014-54, for splitting pre-tax and after-tax amounts and the partial-distribution rule.
• Notice 2013-74 and Notice 2010-84, for in-plan Roth rollovers, withholding, the five-year recapture rule and the ban on unwinding.
• 26 U.S.C. 402A(c)(4), 415(c), 414(v)(3), 401(m) and 72(d)(2), statute text.
• IRS 401(k) Fix-it Guide, ADP and ACP tests, and 26 CFR 1.401(m)-3, for the ACP test, HCE definition and the safe harbor rule.
• IRS Publication 590-B (2025), Roth IRA five-year rules for conversions and plan rollovers.
• IRS, One-participant 401(k) plans.
• TSP, Summary of the Thrift Savings Plan and TSP.gov, Roth in-plan conversions.
• H.R. 5376 as passed by the House (section 138311) and Public Law 117-169 as enacted.

Method notes. All limits are 2026 figures unless labeled 2025. Examples with named people are hypothetical; the 7% return and 24% tax rate are assumptions for illustration. Plan features vary by employer, and nothing here describes a specific plan. 2027 limits had not been published when we checked.

This article is for general information and is not financial or tax advice. Figures are from the IRS, the Thrift Savings Plan and federal statute, checked against the primary sources on 2026-10-08. Whether a mega backdoor Roth works for you depends on your plan document, your pay and your tax situation, so confirm the details with your plan administrator or a tax professional before contributing or converting.

FAQ

What is the mega backdoor Roth limit for 2026?
It is the $72,000 section 415(c) limit minus your elective deferrals and employer contributions. If you defer the full $24,500 and get no employer money, that leaves $47,500. It can never exceed 100% of your pay from that employer.

Do catch-up contributions reduce my mega backdoor room?
No. Catch-up contributions ($8,000 at 50+, $11,250 at ages 60 to 63 in 2026) are outside the 415(c) limit, so they are added on top. That is why the IRS total is $80,000 or $83,250 with catch-ups.

Is there an income limit for the mega backdoor Roth?
No. Neither after-tax 401(k) contributions nor conversions to Roth have an income limit. The practical limit for high earners is the ACP test, which can cap or refund after-tax contributions for highly compensated employees.

Does a Roth conversion of after-tax money count toward my $24,500 deferral limit?
No. The statute says an in-plan Roth rollover is not counted toward the designated Roth contribution limit. Only your own pre-tax and Roth deferrals use the $24,500.

Do my traditional IRA balances make a mega backdoor taxable?
No. The IRA pro-rata rule on Form 8606 applies to conversions out of IRAs. A mega backdoor moves money out of a 401(k), where the plan-level allocation and Notice 2014-54 decide what is taxable.

How do I know if my 401(k) allows a mega backdoor Roth?
Check the summary plan description for "after-tax contributions" plus either "in-plan Roth rollover" or "in-service withdrawals" of after-tax money, or ask the plan administrator. A Roth 401(k) option by itself is not enough.

Can federal employees do a mega backdoor Roth in the TSP?
No. The TSP accepts traditional and Roth employee contributions only, with no after-tax (non-Roth) option. Its Roth in-plan conversion converts pre-tax money and is fully taxable.

Can I undo an in-plan Roth conversion?
No. IRS Notice 2010-84 says in-plan Roth rollovers cannot be unwound, and Roth IRA conversions made after 2017 cannot be recharacterized either.

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"Mega Backdoor Roth: How It Works and the 2026 Limit." Wealthy Pot, 2026. https://wealthypot.com/mega-backdoor-roth/