After-Tax 401(k) Contributions
After-tax 401(k) contributions are voluntary employee contributions to a 401(k) plan made with dollars that have already been subject to income tax, as opposed to pre-tax deferrals (which reduce current taxable income) or Roth 401(k) deferrals (which are also after-tax but grow and distribute tax-free). After-tax contributions enable participants to save beyond the standard deferral limit, forming the basis of the Mega Backdoor Roth strategy when paired with in-plan Roth conversions or in-service distributions.
The annual limit on all additions to a participant's 401(k) account — including employee deferrals, employer match, profit-sharing, and after-tax contributions — is governed by IRC Section 415, which caps total additions at the lesser of 100% of compensation or $70,000 in 2025 (plus the $7,500 catch-up for those 50 and older). The $23,500 elective deferral limit covers only pre-tax and Roth 401(k) deferrals. The gap between the elective deferral limit and the Section 415 limit — roughly $46,500 for most participants in 2025 — can be filled by after-tax contributions if the plan document permits them.
After-tax 401(k) contributions are not immediately tax-deductible and do not reduce current taxable income. The tax treatment is somewhat similar to a non-deductible IRA contribution: the amount contributed is tracked as basis in the plan, and only the earnings on those contributions are taxable upon distribution. However, unlike a traditional IRA, the plan administrator tracks after-tax basis separately from pre-tax amounts in the plan, which is critical for calculating the taxable portion of future distributions.
The primary strategic use of after-tax contributions is the Mega Backdoor Roth. If the plan permits either in-service withdrawals or in-plan Roth conversions, a participant can contribute after-tax dollars up to the Section 415 limit, then immediately convert those funds to Roth status. Because the after-tax contributions have little or no earnings at the time of conversion (assuming the conversion is done promptly), the taxable amount of the conversion is minimal. The converted amount then grows tax-free and can be distributed tax-free in retirement, effectively allowing high-income earners who are above the Roth IRA income limits to accumulate Roth dollars far in excess of the Roth IRA contribution limit.
Not all 401(k) plans allow after-tax contributions. The plan document must specifically authorize them, and the plan must pass nondiscrimination testing on after-tax contributions under the ACP test. For this reason, many large-employer plans offer the feature while smaller plans may not. Participants should confirm with their plan administrator whether after-tax contributions are permitted and whether the plan supports in-service distributions or in-plan Roth conversions before attempting to implement the Mega Backdoor Roth strategy.
Distributions of after-tax contributions are generally tax-free return of basis, but any earnings on those contributions that have not been converted to Roth are taxable as ordinary income upon distribution. The pro-rata rule that complicates backdoor Roth IRA conversions does not apply within a 401(k) in the same way, because the plan tracks after-tax contributions separately from pre-tax amounts, and in-plan Roth conversions can be directed specifically at the after-tax balance.