After-tax 401(k) contributions are taxed once, at the moment you earn them: the money comes out of your paycheck after federal income tax and FICA withholding, so it lands in the plan with tax already paid. From there, the contributions grow tax-deferred. When you eventually take a distribution, your original contributions come back to you tax-free because you already paid tax on them, but every dollar of investment earnings on those contributions is taxed as ordinary income in the year you withdraw it.
That is the short answer. The details matter, because the IRS applies a specific accounting rule to every distribution, imposes a penalty on early withdrawals of the earnings, and offers a rollover path that can make the earnings permanently tax-free if you act in time.
Contributions Go In With After-Tax Dollars
When you elect after-tax contributions, your employer withholds federal income tax and FICA from that portion of your pay before depositing it into the plan.1Internal Revenue Service. Retirement Plan FAQs Regarding Contributions Unlike traditional pre-tax deferrals, these contributions do not reduce your adjusted gross income for the year.2Internal Revenue Service. 401(k) Plan Overview There is no upfront deduction, no reduction in taxable wages on your W-2, no tax break in the contribution year.
What you get in exchange is basis. The IRS treats those already-taxed dollars as your investment in the plan, and your plan administrator tracks the running total so the same money is never taxed twice. That basis is what makes the eventual distribution math work in your favor.
Earnings Grow Tax-Deferred, Then Are Taxed as Ordinary Income
Once your after-tax dollars are inside the plan, dividends, interest, and capital gains compound without appearing on your annual return.3eCFR. 26 CFR 1.402(a)-1 In a taxable brokerage account, you would owe tax each year on that growth. Inside the 401(k), the full balance keeps compounding without that annual drag.
The trade-off shows up at withdrawal. All earnings on after-tax contributions come out as ordinary income, not as long-term capital gains. You pay your regular marginal rate, which for many people is higher than the 0%, 15%, or 20% rate that would have applied to long-term gains in a taxable account. Years of tax-deferred compounding sit on one side of the scale; ordinary-income treatment on the growth sits on the other.
Every Distribution Mixes Basis and Earnings
You cannot pull only the tax-free basis and leave the taxable earnings in the plan. The IRS requires every distribution from your after-tax sub-account to include a proportional share of basis and earnings, calculated from the ratio of each to the current balance.4Office of the Law Revision Counsel. 26 USC 72
An example makes the math concrete. Suppose your after-tax sub-account holds $10,000, split between $8,000 of basis and $2,000 of earnings. Earnings are 20% of the balance. If you take $1,000, then $800 is a tax-free return of basis and $200 is taxable ordinary income. The ratio recalculates with each distribution as the balance changes. If you ignore the split when estimating your tax bill, you can end up with an underpayment penalty.
Your plan administrator reports the breakdown on Form 1099-R. Your after-tax basis appears in Box 5, labeled “Employee Contributions,” and the taxable portion appears in Box 2a.5Internal Revenue Service. Instructions for Forms 1099-R and 5498 If those numbers look wrong, contact your plan administrator before you file. Errors in the 1099-R are tedious to unwind once the IRS has the mismatched copy.
Early Distributions Trigger a 10% Penalty on the Earnings
If you take a distribution before age 59½, the taxable portion is hit with a 10% additional tax on top of ordinary income tax.6Internal Revenue Service. Topic No. 558 Only the earnings are exposed. Your basis comes back penalty-free at any age because the tax was already paid. But because of the pro-rata rule, every early distribution contains some earnings, so some portion always faces both regular income tax and the 10% penalty.
Several exceptions can waive the 10% penalty before 59½, including disability, separation from service after age 55, and substantially equal periodic payments. Those exceptions apply only to the additional tax. Regular income tax on the earnings portion is owed regardless.
Required Minimum Distributions Apply Too
After-tax balances inside a 401(k) are subject to required minimum distributions like the rest of the plan.7Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs For 2026, RMDs generally must begin by April 1 of the year after you turn 73, or, if the plan allows, April 1 of the year after you retire, whichever is later.8Internal Revenue Service. RMD Comparison Chart If you own more than 5% of the company, RMDs start at 73 whether or not you are still working.
Each RMD carries the same pro-rata treatment. Part is a tax-free return of basis, part is taxable earnings. That forced taxable slice is one reason many people move after-tax dollars out to a Roth IRA before RMD age. Roth IRAs have no RMDs during the owner’s lifetime.
Rolling the Basis to a Roth Changes the Tax Outcome
The most valuable feature of after-tax contributions is what you can do with the basis on the way out. IRS Notice 2014-54 allows you to split a distribution and send the components to different destinations: the after-tax basis to a Roth IRA, and the pre-tax amounts (which include the earnings on your after-tax contributions) to a traditional IRA.9Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans10Internal Revenue Service. Notice 2014-54
Moving the basis to a Roth IRA triggers no additional tax, because the tax was already paid on it. Once inside the Roth IRA, future growth becomes tax-free if you meet two conditions for a qualified distribution: you are at least 59½, and the Roth IRA has been open for at least five taxable years.11Office of the Law Revision Counsel. 26 USC 408A The five-year clock starts on January 1 of the first year you contributed to any Roth IRA, so a long-standing Roth IRA has that clock already running.
Timing is the lever that controls your tax bill on this move. The longer after-tax contributions sit in the plan before you convert them, the more earnings pile up, and those earnings are taxed as ordinary income if you send them into a Roth. Convert $20,000 of after-tax contributions that have grown by $500, and $500 is taxable in the conversion year. Converting soon after each payroll contribution keeps the taxable amount small, sometimes near zero. Some plans automate this with in-plan conversions, which is the cleanest version.
If your plan does not allow in-service withdrawals or in-plan Roth conversions, the rollover option is generally not available while you are still employed. You can wait until you leave the employer or reach the plan’s distribution age, then request a full distribution and split it between a Roth IRA (for the basis) and a traditional IRA (for the pre-tax amounts). A partial distribution of only the after-tax money is not permitted; any partial distribution has to include a proportional share of pre-tax dollars, so cleanly separating the two usually requires a full distribution.
Not the Same as Roth 401(k) Contributions
After-tax contributions and designated Roth 401(k) contributions are both funded with dollars you have already paid income tax on, and the similarity ends there. Roth contributions are elective deferrals that count against the annual deferral limit, and qualified withdrawals, including the earnings, come out completely tax-free. After-tax contributions sit in a separate bucket outside the deferral limit, and the earnings on them are taxable at ordinary rates when distributed. Confusing the two leads to the wrong assumption about how the growth will be taxed later, which is precisely where the rollover strategy above becomes worth understanding.