After-Tax 401(k) Contributions and In-Plan Roth Conversions

Inside roughly half of large-employer 401(k) plans is a feature most participants never use: the after-tax contribution bucket. Combined with an in-plan Roth conversion or in-service rollover, it is the mechanism behind the mega backdoor Roth — up to $40,000+ of additional Roth savings per year for employees who can fund it.

The three buckets inside one 401(k)

A single 401(k) account can hold three legally distinct contribution sources:

• Pre-tax elective deferrals — capped at $23,500 (2025) under IRC §402(g)
• Roth elective deferrals — sharing the same $23,500 cap
• After-tax employee contributions — different from Roth, governed by §401(m); fill the gap between deferrals + match and the §415(c) total of $70,000

The after-tax bucket is what the mega backdoor depends on. Without it, the strategy is impossible.

The headroom math

The §415(c) annual additions limit for 2025 is $70,000. Subtract from it:

• Your elective deferral (typically the full $23,500)
• Your employer match (varies — assume $7,000)

The remainder — about $39,500 in this example — is the maximum after-tax contribution. Convert it to Roth treatment and it grows tax-free for life.

The two paths to Roth conversion

An after-tax contribution sitting in the plan is still subject to ordinary-income tax on its earnings when distributed. The strategy is to move it to Roth status as quickly as possible so the earnings start compounding tax-free. Two mechanisms:

• In-plan Roth conversion under IRC §402A(c)(4). The plan recharacterizes the after-tax balance as Roth inside the plan. Authorized by SECURE Act of 2019 and now common.
• In-service rollover to a Roth IRA. While still employed, you take an in-service distribution of the after-tax portion and roll it directly to your Roth IRA. IRS Notice 2014-54 confirmed that pre-tax earnings can be split to the Traditional IRA while after-tax basis goes to the Roth — no pro-rata blending.

Worked example

An engineer earning $300,000 at an employer with a $7,000 match contributes:

• Pre-tax 401(k) deferral: $23,500
• Employer match: $7,000
• After-tax contribution: $70,000 − $23,500 − $7,000 = $39,500
• Total annual addition: $70,000 (the §415(c) cap)

The $39,500 is converted to Roth in-plan the same year. Over a 20-year career at 7% growth, that single year's mega backdoor adds about $153,000 of tax-free wealth — and the strategy is repeatable annually.

Questions to ask HR

1. "Does the plan permit after-tax employee contributions separate from Roth deferrals?"
2. "Are in-plan Roth conversions allowed?"
3. "Does the plan permit in-service withdrawals of after-tax contributions?"
4. "What is the timing — daily, monthly, or annual conversion windows?"

"Yes" to questions 1 and (2 or 3) unlocks the strategy. Anything else and the after-tax money grows tax-deferred but the earnings are taxable on distribution — still useful, but a fraction of the mega backdoor benefit.

The ADP/ACP test risk

After-tax contributions are subject to the §401(m) ACP test. Plans without a safe harbor or with low non-highly-compensated participation may refund HCE after-tax contributions in the spring of the following year if the test fails. Confirm the plan's testing history before assuming the full $39,500 will stick.

Common mistakes

• Confusing after-tax with Roth. Different §415(c) treatment, different conversion path, different ACP testing. Ask explicitly for after-tax, not "Roth."
• Delaying conversion. Earnings on un-converted after-tax dollars are taxable. Convert at every available window — daily if the plan supports it.
• Forgetting the match in the headroom calculation. A generous match (10%+ of salary) substantially shrinks the after-tax bucket. Run the math for your specific match.
• Assuming all big-company plans offer it. They don't. The Plan Sponsor Council of America's 2024 survey reports about 50% of plans permit after-tax contributions, and a smaller share permit in-plan Roth conversions.

Sources

• Internal Revenue Code §415(c), annual additions limit (Cornell LII): law.cornell.edu/uscode/text/26/415
• Internal Revenue Code §402A(c)(4), in-plan Roth conversions: law.cornell.edu/uscode/text/26/402A
• IRS Notice 2014-54, allocation of after-tax amounts to multiple destinations: irs.gov/pub/irs-drop/n-14-54.pdf
• Internal Revenue Code §401(m), ACP test for after-tax and match contributions: law.cornell.edu/uscode/text/26/401
• Plan Sponsor Council of America, 67th Annual Survey of Profit Sharing and 401(k) Plans (2024).

RetirementCheck101 sizes your mega backdoor headroom in step 3. Explore the free educational tool to see what your plan permits.