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

After-tax 401(k) contributions are a third bucket in your workplace plan that lets you save beyond the standard elective deferral cap, up to a combined plan ceiling of $72,000 in 2026. They use money you’ve already paid income tax on, and their real value comes from being converted into a Roth account, a move often called the mega backdoor Roth. Whether you can use them at all depends on your specific plan document.

How They Differ From Pre-Tax and Roth

Your 401(k) can hold money in three separate buckets. Pre-tax contributions lower your taxable income today and are fully taxed on withdrawal. Roth contributions come from post-tax pay, and qualified withdrawals are tax-free. Voluntary after-tax contributions also use post-tax dollars, but they don’t share the Roth’s tax-free growth. Earnings on after-tax money are taxed as ordinary income when withdrawn.

That gap is the whole reason to move after-tax dollars out quickly. The longer they sit, the more taxable earnings pile up on top of the already-taxed principal. Converting to a Roth IRA or a Roth 401(k) locks in tax-free growth on the principal going forward and keeps the taxable earnings component small.

One thing catches people off guard: after-tax contributions usually don’t get an employer match. Most plans only match elective deferrals. Check your plan document, but don’t assume.

2026 Contribution Limits

The IRS caps everything that flows into a defined contribution plan under Section 415(c). For 2026, the ceiling is $72,000 per participant under age 50.¹ That $72,000 includes your pre-tax and Roth deferrals, your employer’s matching and profit-sharing dollars, and your voluntary after-tax contributions combined.

The elective deferral limit for 2026 is $24,500.² Your after-tax room is whatever is left of the $72,000 after subtracting your deferrals and your employer’s contributions. If you defer the full $24,500 and your employer adds $10,000 in matching, you have $37,500 of after-tax capacity.

Catch-up contributions sit on top of the $72,000 ceiling rather than reducing your after-tax room:

Ages 50–59 and 64 and older can add $8,000, for a total plan limit of $80,000.²
Ages 60 through 63 get an enhanced catch-up of $11,250 under SECURE 2.0, for a total of $83,250.²

401(k) money goes in through payroll each pay period. You can’t make a lump-sum after-tax deposit at year-end, so if you want to hit your target, you need to set the deduction percentage early enough for payroll to spread it across the remaining paychecks.

Does Your Plan Allow the Mega Backdoor Roth?

The mega backdoor Roth is the informal name for making after-tax contributions and then converting them to a Roth account. Since the principal has already been taxed, converting it triggers little or no additional tax on that portion. Only the earnings that accrued before the conversion get taxed.

Two plan features have to exist for the strategy to work:

The plan must explicitly permit voluntary after-tax contributions beyond the elective deferral limit.
The plan must allow either in-service distributions (so you can roll after-tax funds to an outside Roth IRA while still employed) or in-plan Roth conversions (moving the money into the Roth 401(k) bucket internally).

Without both features, after-tax dollars just sit in the plan collecting taxable earnings until you leave the employer or reach a qualifying event, which defeats the purpose. Read your Summary Plan Description or call the plan administrator before you set up payroll deductions.

Moving the Money to a Roth

IRS Notice 2014-54 is the guidance that makes the split work. When a distribution contains both pre-tax and after-tax amounts and you direct it to multiple destinations at once, the IRS treats it as a single distribution for allocation.³ In practice, you can send the pre-tax earnings to a traditional IRA and the after-tax principal to a Roth IRA in one coordinated move. The pre-tax dollars keep their tax deferral in the traditional IRA, and the after-tax basis lands in the Roth IRA with no tax on the principal.

To start the transfer, you’ll need the destination account numbers, a distribution request form from your plan administrator, and the receiving custodians’ wiring or mailing instructions. Plans that offer in-plan Roth conversions are simpler because the money stays with the same provider and just changes buckets internally.

If your plan requires spousal consent for distributions, that requirement kicks in when the lump-sum value exceeds $5,000.⁴ Mega backdoor Roth amounts almost always clear that threshold, so get the waiver signed before submitting. Plan administrators typically need two to four weeks to process a rollover from start to finish.

The 20% Withholding Trap

If the plan sends the distribution check to you instead of transferring it directly to the receiving IRA or plan, the administrator must withhold 20% of the taxable portion for federal income tax.⁵ Withholding does not apply when funds move directly to another retirement plan or IRA.⁶

Here’s why that matters. If you receive the check yourself, you have 60 days to deposit the full original distribution amount into a qualifying account to avoid taxes and penalties.⁶ But 20% is already gone to withholding, so you’d have to cover that gap out of pocket to complete a full rollover. Any shortfall is treated as a taxable distribution. Always request a direct rollover.

How Principal and Earnings Are Taxed

After-tax principal and its earnings follow different rules, and the plan is required to track them separately.

Principal moves to a Roth IRA with no additional tax. Earnings on after-tax contributions, however, are classified as pre-tax amounts.⁷ If you take those earnings as cash rather than rolling them, they’re taxed as ordinary income, and withdrawing them before age 59½ adds a 10% early distribution penalty.

The Pro-Rata Rule

You can’t cherry-pick only the after-tax basis for a partial distribution. Any partial withdrawal from a mixed account must include a proportional share of both pre-tax and after-tax amounts.⁷ On a $100,000 account with $80,000 pre-tax and $20,000 after-tax, a $50,000 distribution would consist of $40,000 pre-tax and $10,000 after-tax.

That is exactly why Notice 2014-54 matters. The pro-rata rule dictates what comes out; Notice 2014-54 lets you point each piece to a different destination in the same transaction.³

Form 1099-R Box 5

Your plan reports the distribution on Form 1099-R at year-end. Box 5 shows your after-tax basis, meaning the employee contributions that were already included in income when made.⁸ That figure tells the IRS how much of the rollover is non-taxable. If the number doesn’t match your records, contact your plan administrator before filing, because an error in Box 5 can cause the IRS to treat tax-free principal as taxable income.

If You’re a Highly Compensated Employee

If you earned $160,000 or more from your employer in the prior year, you’re classified as a highly compensated employee for 2026.¹ Your after-tax contributions then fall under the Actual Contribution Percentage test, which compares HCE contribution rates against those of everyone else in the plan.⁹

When rank-and-file participation is low, that math effectively caps HCE after-tax contributions well below the theoretical 415(c) space. If the plan fails the test, excess contributions are returned to the affected HCEs. Some employers use a safe harbor design that avoids ACP testing altogether, which is the best-case scenario for a high earner trying to max after-tax contributions.

Correcting Excess Contributions

If total contributions exceed the 415(c) limit, the plan has to correct the error to stay qualified, following an IRS-specified order that starts with returning unmatched elective deferrals and works through matched deferrals and employer contributions until the account is back within limits.¹⁰

Excess elective deferrals over the $24,500 cap have a stricter deadline. The corrective distribution must go out by April 15 of the year following the excess, and a tax return extension does not push that date back.¹¹ Miss it and the excess is taxed twice, once in the year contributed and again when eventually distributed, and you lose the ability to roll a corrective distribution to another plan or IRA.¹⁰

Looking Ahead to 2027

Starting in 2027, SECURE 2.0 will require catch-up contributions to be made as Roth for workers who earned $150,000 or more in FICA wages from that employer in the prior year.¹² This does not apply in 2026, but if you’re currently making pre-tax catch-up contributions above that threshold, expect the change. Plans without a Roth option will need to add one, or their high-earning participants will lose catch-up eligibility.

1
Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
2
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
3
Internal Revenue Service. Notice 2014-54 – Guidance on Allocation of After-Tax Amounts to Rollovers
4
Internal Revenue Service. Fixing Common Plan Mistakes – Failure to Obtain Spousal Consent
5
eCFR. 26 CFR 31.3405(c)-1 – Withholding on Eligible Rollover Distributions
6
Internal Revenue Service. Rollovers of Retirement Plan and IRA Distributions
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. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
10
Internal Revenue Service. Fixing Common Plan Mistakes – Failure to Limit Contributions for a Participant
11
Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
12
Internal Revenue Service. Treasury, IRS Issue Final Regulations on New Roth Catch-Up Rule, Other SECURE 2.0 Act Provisions