Use non-Roth after-tax contributions, not just a Roth 401(k)
Mega backdoor Roth is an informal name for moving non-Roth after-tax workplace-plan contributions to Roth. It is not a new, independent tax-free contribution allowance. This article covers U.S. federal tax for U.S. citizens and U.S. tax residents using a regular 401(k). It does not substitute for Japan’s NISA or iDeCo rules or nonresident tax treatment.
The IRS workplace Roth guidance and after-tax rollover guidance distinguish the following three contribution types. A workplace that offers Roth may still not offer the third row.
| Contribution type | Contribution and earnings treatment | Relationship to employee deferral limit |
|---|---|---|
| Pre-tax elective deferrals | Federal income tax is deferred at contribution | Shared $24,500 limit |
| Roth 401(k) elective deferrals | Taxed at contribution; tax-free earnings require a qualified distribution | The same shared $24,500 limit |
| Non-Roth after-tax contributions | Contributions are already taxed, but earnings are pre-tax amounts | Outside the deferral limit but inside annual additions |
A tax allowance does not create an investment profit. Assets can fall in value after a Roth transfer.
Two plan features to establish first
The IRS designated Roth account FAQs (In-plan Roth rollovers) explain that an in-plan Roth rollover requires plan authorization. A plan can permit direct transfers of otherwise nondistributable amounts, but this is not an automatic feature of every plan.
- Does the plan accept non-Roth after-tax contributions? Roth elective deferrals alone are insufficient.
- For that balance, does it permit an in-plan Roth rollover, or a distribution eligible for an external rollover to a Roth IRA or other appropriate account?
If external in-service rollovers are unavailable, a later event such as leaving employment may create distribution eligibility. No in-service distribution does not necessarily mean no future rollover. Before starting, obtain the Summary Plan Description (SPD) and administrator confirmation of eligible balances, frequency, fees, automation and contribution caps.
In addition to the statutory limits in the IRS 401(k) contribution guidance, plan restrictions or nondiscrimination testing may reduce the available amount. A contribution option on the account screen does not establish eligibility for the maximum.
Subtract employer contributions when calculating 2026 room
IRS Notice 2025-67 (pages 1–2) sets the regular 2026 employee elective-deferral limit at $24,500 and the defined-contribution annual-additions limit at $72,000. The IRS annual-additions explanation limits the latter to 100% of compensation if that is lower. Eligible catch-ups are outside annual additions: generally $8,000 at age 50 or older, or $11,250 instead for those attaining ages 60–63 during the year.
Assumptions: under age 50, sufficient compensation, $24,500 in employee elective deferrals, no other annual additions, and a plan permitting after-tax contributions up to the remaining room. Room = $72,000 − regular employee deferrals − employer contributions.
| Hypothetical employer contribution | Remaining-room calculation | Potential after-tax limit |
|---|---|---|
| None | $72,000−$24,500−$0 | $47,500 |
| 4% of $120,000 salary = $4,800 | $72,000−$24,500−$4,800 | $42,700 |
| 4% of $200,000 salary = $8,000 | $72,000−$24,500−$8,000 | $39,500 |
Actual calculations also subtract other annual additions, such as extra profit-sharing contributions or allocated forfeitures. Subtracting an eligible age-50+ catch-up from $72,000 again understates room. Reserve for expected employer year-end contributions until the administrator confirms them. Notice 2025-67 also specifies a $150,000 prior-year wage threshold for determining the 2026 Roth catch-up requirement, so confirm the Roth requirement with payroll, including whether prior-year applicable wages from the sponsoring employer exceeded that amount.
Pro-rata allocation still matters: separate basis and earnings
The IRS after-tax rollover guidance (partial distributions and multiple destinations) says distributions from an account containing pre-tax and after-tax amounts generally include a proportional share of both. It is incorrect to say that after-tax status lets you select only basis without allocation. Confirm the relevant account and distribution boundaries with the administrator.
An eligible distribution sent simultaneously to multiple destinations can allocate after-tax basis to a Roth IRA and pre-tax amounts to a traditional IRA or other appropriate plan. Earnings on after-tax contributions are pre-tax amounts too. This allocates the components of the distribution to destinations; it does not exempt a partial distribution from pro-rata treatment.
| Assumed eligible distribution: $10,500 = $10,000 taxed basis + $500 earnings | Treatment in the rollover year |
|---|---|
| Move the entire amount to Roth | Do not tax the $10,000 basis again; include the $500 pre-tax amount in income |
| Properly directly roll basis to Roth IRA and earnings to traditional IRA | Defer tax on $500 of earnings; future withdrawals or other events can trigger tax |
If the $500 earnings moved to Roth incur additional federal income tax at 24%, the calculation is $500×24% = $120. This assumes a constant tax rate and excludes state taxes and other effects. It is not a forecast of earnings based on conversion frequency.
The IRS in-plan Roth rollover FAQs likewise calculate taxable income by subtracting already-taxed basis from the amount transferred. Earlier transfers may reduce the buildup of subsequent earnings, but existing pre-tax amounts and market changes prevent a blanket promise of zero tax.
Reconcile records before and after a transfer
- Obtain the administrator’s breakdown of after-tax basis, associated earnings and other pre-tax amounts.
- Confirm eligible direct-rollover amounts, amounts going to Roth and traditional destinations, and the receiving accounts.
- Reconcile Form 1099-R with account transactions and check the reporting instructions for the relevant year.
- If pre-tax funds move into an external traditional IRA, ask a tax professional how that affects any separate IRA-to-Roth conversion.
The IRS Roth account guidance explains qualified-distribution requirements involving five years and age or other conditions; the Roth FAQs also explain early-withdrawal treatment following in-plan rollovers. Completing a rollover does not make all funds, including earnings, immediately accessible tax-free. Separately assess cash for any tax bill and money that can remain committed until retirement.
To establish ordinary employee contribution limits first, see the IRA and 401(k) shared-limit and deduction comparison.
This is general information. Consult a U.S. tax professional about your individual circumstances. Investment disclaimer
References
5- Notice 2025-67, 2026 limits, pages 1–5 (opens in a new tab) IRS · 2026 tax year · checked 2026-09-07
- 401(k) and profit-sharing plan contribution limits: 2026 (opens in a new tab) IRS · checked 2026-09-07
- Designated Roth account FAQs: In-plan Roth rollovers (opens in a new tab) IRS · checked 2026-09-07
- Roth account in your retirement plan: Qualified distributions (opens in a new tab) IRS · checked 2026-09-07
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