After-Tax 401(k) Contributions: Limits, Mega Backdoor Roth, Rollovers

After-tax 401(k) contributions are dollars you put into your employer’s retirement plan from pay you’ve already been taxed on, deposited above the normal elective deferral cap and up to a combined annual ceiling of $72,000 in 2026. They exist mainly as a setup move: once the money is in the plan, you convert it to a Roth account so future growth comes out tax-free in retirement. That conversion is the whole point for most people who use them, and it’s commonly called the mega backdoor Roth.

How They Differ From Roth 401(k) Deferrals

These are not the same thing as Roth 401(k) contributions, and mixing them up will cost you room. Roth deferrals are elective deferrals. They share the same $24,500 annual cap for 2026 that applies to traditional pre-tax deferrals.¹ Put $24,500 into Roth deferrals and you’ve used your entire deferral limit.²

Voluntary after-tax contributions sit outside that cap entirely. They don’t touch your $24,500. They fill the space between what you and your employer have already put in and the much higher overall plan limit. Both types come from after-tax pay, but Roth deferrals get tax-free-growth treatment automatically. After-tax contributions only get that treatment if you convert them, and the conversion is a step you have to take.

2026 Contribution Limits

Two separate IRS limits control your 401(k) each year, and your after-tax room is the gap between them.

The elective deferral limit under Section 402(g) is $24,500 for 2026. It covers the combined total of your pre-tax and Roth deferrals.¹

The annual additions limit under Section 415(c) is $72,000 for 2026. It covers everything going into your account: your deferrals, employer matching and profit-sharing, and your voluntary after-tax contributions.³ The statutory definition of “annual addition” explicitly includes both employer and employee contributions.⁴

Your after-tax room is whatever’s left after subtracting deferrals and employer contributions from $72,000. If you defer $24,500 and your employer adds $12,000 in matching, you have $35,500 of after-tax space ($72,000 − $24,500 − $12,000).

Catch-Up Contributions for Older Workers

Catch-up contributions raise your deferral limit but sit outside the $72,000 annual additions cap. The standard catch-up for participants age 50 and older is $8,000 in 2026, bringing the maximum deferral to $32,500.¹

SECURE 2.0 created an enhanced catch-up starting in 2025 for participants turning 60, 61, 62, or 63 during the calendar year. For 2026 that amount is $11,250, allowing total deferrals of $35,750.¹ Because catch-ups don’t count toward the $72,000 ceiling, your after-tax room stays the same regardless of age.

Whether Your Plan Allows Them

Not every 401(k) offers this. The feature has to be written into the plan document, and it’s at the employer’s discretion. Start with your Summary Plan Description, which should list voluntary after-tax contributions as a contribution type if the option exists. If the document is unclear, ask your plan administrator two questions: does the plan accept voluntary after-tax contributions, and does it allow either in-service distributions or in-plan Roth conversions of those contributions?

Both matter. A plan that accepts after-tax contributions but has no conversion mechanism gives you tax-deferred growth on earnings and nothing more, which strips out most of the strategic value. Without in-service conversions, you’d have to wait until you leave the employer before rolling the money into a Roth IRA.

When you enroll, the election form is usually separate from your standard deferral election. You set a contribution amount as either a percentage of pay or a flat dollar figure per pay period. Watch your next couple of pay stubs for a line item labeled as an after-tax or non-Roth post-tax deduction. If the amount doesn’t match what you elected, contact benefits immediately, because payroll errors here can cascade into over-contribution problems.

The Mega Backdoor Roth Conversion

Once the after-tax money is in the plan, you move it into Roth. That’s what makes this worth doing. Two paths get you there.

An in-plan Roth rollover moves the after-tax dollars from the after-tax subaccount to a designated Roth account inside the same 401(k). The plan trustee handles the transfer internally.⁵
An in-service distribution to a Roth IRA rolls the after-tax contributions out of the 401(k) entirely and into your external Roth IRA. This requires the plan to allow in-service distributions while you’re still employed.

Both achieve the same tax result. You pay tax only on any earnings that accrued in the after-tax subaccount before the conversion. The principal itself was already taxed, so converting it costs nothing. Speed matters. Convert quickly, before investment gains pile up and create a tax bill. Some plans allow automatic conversion of after-tax contributions at regular intervals, which removes the need to request each transfer.

The Five-Year Recapture Rule

In-plan Roth rollovers carry a wrinkle. If you withdraw any amount from the converted Roth balance within five tax years of the conversion, the 10% early distribution penalty can apply to the taxable portion, even though the original after-tax principal wouldn’t normally be penalized. The clock resets with each conversion, so frequent conversions create multiple overlapping five-year windows.⁵ For long-term retirement savings the clock is irrelevant. It matters if you might need the money sooner.

How Principal and Earnings Are Taxed

After-tax contributions have a split identity for tax purposes. The principal has already been taxed through payroll, so you won’t owe income tax on those dollars again when you withdraw or convert them. Your basis is dollar-for-dollar what you put in.

Earnings on those contributions are different. Investment gains inside the after-tax subaccount grow tax-deferred, not tax-free. When those earnings come out, whether through a distribution, a conversion, or a rollover, they’re taxed as ordinary income. That gap is the core reason to convert to Roth: once converted, future earnings on that money grow and come out tax-free.

When you move an after-tax balance to multiple destinations at once, IRS Notice 2014-54 lets you direct the after-tax principal to a Roth IRA and the pre-tax earnings to a traditional IRA in a single distribution. The IRS treats the simultaneous rollover as one distribution for allocation purposes, so the two pieces don’t have to be blended.⁶⁷ Say your after-tax subaccount holds $30,000 in contributions and $2,000 in earnings. Split the distribution: the $30,000 goes to the Roth IRA tax-free, and the $2,000 rolls to a traditional IRA and stays tax-deferred until you withdraw it later.

Your plan administrator should track after-tax contributions separately from pre-tax amounts and earnings. When you take a distribution, the plan reports the taxable and non-taxable portions. If those records are wrong, you could end up paying tax on money you already paid tax on, and fixing it after the fact is painful.

Limits for Highly Compensated Employees

Even if your plan offers after-tax contributions and you have room under the $72,000 cap, non-discrimination testing can cut your actual limit. The IRS requires 401(k) plans to pass the Actual Contribution Percentage (ACP) test, which compares after-tax and matching contribution rates between highly compensated employees and everyone else.⁸

For 2026 you’re a highly compensated employee if you earned more than $160,000 from the employer in the prior year.³ The ACP test caps HCE contribution rates based on what non-highly compensated employees contribute. When rank-and-file employees aren’t making after-tax contributions, and most don’t, the test can severely restrict or eliminate the after-tax option for higher earners.

A safe harbor plan design is automatically deemed to pass the ACP test for matching contributions, but that exemption doesn’t extend to voluntary after-tax contributions.⁹ If the plan fails the test, the employer distributes the excess back to HCEs. Those refunds are taxable in the year received and can’t be rolled over.⁸ Some administrators cap after-tax contributions for HCEs before that becomes an issue, which is why your internal plan limit may be lower than the IRS maximum.

Rolling the Balance Out When You Leave

Separation from your employer opens full access to roll the after-tax balance. The same Notice 2014-54 splitting rules apply, so you can direct the principal to a Roth IRA and the earnings to a traditional IRA in a single distribution.⁷ If your plan never allowed in-service conversions, this is your first chance to move those dollars into Roth territory.

Request a direct rollover rather than taking a check. Direct rollovers avoid the mandatory 20% withholding and the 60-day deadline. Take the distribution as cash and the plan withholds 20% of the taxable portion, and you have 60 days to deposit the full amount, including making up the withheld piece out of pocket, into the destination accounts.⁵ Miss the 60-day window and the earnings become taxable, plus a potential 10% early distribution penalty if you’re under 59½.

If You Contribute Too Much

When total annual additions exceed the $72,000 limit, the plan has to correct the excess. For elective deferrals over the separate $24,500 cap, you request a return of the excess plus earnings by April 15 of the following year. Miss that deadline and the excess is taxed twice, once in the year of deferral and again when distributed.¹⁰

For after-tax contributions that push total additions past the 415(c) ceiling, correction typically falls on the plan administrator. The plan may return the excess or reallocate it. Staying under the limits yourself is easier than cleaning up afterward, particularly because corrections that drag past the plan year’s deadline can jeopardize the plan’s tax-qualified status. If you contribute to more than one employer’s plan in the same year, tracking the combined totals is on you, because each employer only sees its own numbers.

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Office of the Law Revision Counsel. 26 USC 402A – Optional Treatment of Elective Deferrals as Roth Contributions
3
Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
4
Office of the Law Revision Counsel. 26 USC 415 – Limitations on Benefits and Contribution Under Qualified Plans
5
Internal Revenue Service. In-Plan Roth Rollovers
6
Internal Revenue Service. IRS Notice 2014-54 – Guidance on Allocation of After-Tax Amounts to Rollovers
7
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
8
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
9
Internal Revenue Service. Notice Requirement for a Safe Harbor 401(k) or 401(m) Plan
10
Internal Revenue Service. Consequences to a Participant Who Makes Excess Annual Salary Deferrals