Educational information, not individual financial advice.
Key Takeaways
The "mega-backdoor Roth" is one of the most powerful tax-deferred-to-Roth conversion strategies available, and one of the most under-used. It's been around since 2014, but most plan participants don't know their plan allows it — or they do, and the HR rep can't explain it.
Your 401(k) has three potential contribution buckets:
The total of all three is capped by § 415(c) — $72,000 in 2026 ($79,500 with 50+ catch-up; $83,250 with the new 60-63 super catch-up). The after-tax bucket is what's left after employee + employer fill their portions.
Worked example, 2026, age 35:
That $38,500 can go into the after-tax bucket, then immediately convert to Roth via either an in-plan Roth conversion or an in-service rollover to a Roth IRA. After conversion: it's Roth dollars. Tax-free growth, tax-free withdrawal in retirement.
The regular backdoor Roth lets high earners (above the Roth IRA income limit) sneak $7,000/yr into a Roth IRA via a non-deductible Traditional IRA contribution + immediate conversion. Useful but small.
The mega-backdoor uses the much larger 401(k) headroom — 5–6× the regular backdoor limit. For a high earner with no employer match, the strategy can put nearly $50k into Roth in a single year.
Three plan features are required:
If your plan has all three, you're golden. If it has after-tax contributions but no in-service conversion, you can still do it — just wait until separation/retirement to convert.
If your plan doesn't allow after-tax contributions, this strategy isn't available to you. (Most large-employer plans allow it; many small-employer plans don't.)
In-plan Roth conversion. Move dollars from the after-tax bucket into the Roth bucket of the same 401(k) plan. Stays inside the 401(k); no rollover paperwork. Subject to plan rules.
In-service rollover to Roth IRA. Move dollars from the after-tax bucket into a separate Roth IRA. Out of the 401(k); the dollars now follow Roth IRA rules (more flexible withdrawal rules, no RMDs).
The Roth IRA path is generally preferred for the more flexible rules, but the in-plan path is fine if your plan offers strong investment options.
If you contribute after-tax and DON'T convert immediately, the after-tax dollars start earning. Earnings on after-tax dollars are pre-tax — they're treated like Traditional 401(k) earnings. On conversion, those earnings are taxable as ordinary income.
Solution: contribute → convert immediately (or as quickly as the plan allows). Some plans offer "automatic conversion" daily or weekly, which keeps the taxable earnings to ~$0. If your plan doesn't, manually trigger the conversion as soon as the contribution settles.
The contribution itself uses after-tax dollars (you've already paid income tax on them). The conversion is therefore non-taxable for the principal. Only the earnings between contribution and conversion are taxable.
This is dramatically different from a Traditional → Roth conversion, where the entire converted amount is taxable. The mega-backdoor's appeal is that you're moving already-taxed dollars into Roth without any new tax bill (assuming negligible earnings between contribution and conversion).
If you're not maxing out the simpler options first (regular 401(k), HSA, Roth IRA via backdoor), do those before the mega-backdoor. The marginal benefit of the mega-backdoor over a taxable brokerage account is large, but the marginal benefit of basic 401(k) over taxable is also large — start there.
Plan limits. Some plans cap after-tax contributions at a fraction of comp (e.g., 10%) below the 415(c) headroom. Read the SPD.
Highly Compensated Employee (HCE) testing. Some plans flunk ACP non-discrimination tests when too many HCEs hit the after-tax limit. The plan can refund excess contributions plus earnings late in the year. Most large-employer plans have safe-harbor designs that pass automatically.
Roth IRA aggregation rules (if rolling out). The Roth IRA five-year rule applies separately to converted dollars. Tracking the rollover-source basis matters if you might withdraw before 59½.
The Horizons engine supports modeling after-tax 401(k) contributions + in-plan Roth conversion via the asset-level account_subtype + contribution_tax_fraction fields. The /retirement page projection accounts for both buckets when computing your projected retirement income mix. Talk to your tax pro before electing — plan rules vary, and HCE testing can refund contributions you'd planned to convert.
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Known limitations
Sources
Educational information distilled from the Horizons engine methodology — not individual financial advice.
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