Mega-Backdoor Roth: In-Plan Convert vs Roll-Out

Priya, a 38-year-old software engineer at a Seattle tech company, files single and earns $235,000. Her 401(k) allows after-tax contributions and both in-plan Roth conversion and in-service distributions — the rare plan that supports the full mega-backdoor Roth. In 2026 she maxes her $24,500 employee deferral (IRC §402(g)), her employer adds a $1,500 match, and that leaves $46,000 of after-tax headroom under the $72,000 total §415(c) limit ($24,500 + $1,500 + $46,000 = $72,000). She contributes the full $46,000 of after-tax money across the year. Now the question nobody answered for her: convert it inside the plan, or roll it out to a Roth IRA at Vanguard?

For Priya, the in-plan conversion is the right default. It moves all $46,000 into her Roth 401(k) sub-account with two clicks, costs nothing, keeps her money under ERISA creditor protection, and — the part that actually decides it — never creates a traditional IRA balance that would wreck the backdoor Roth she also runs each year. She should roll out only if she has a specific fund she can’t buy inside the plan. Here is the full decision.

The two routes, precisely

Both routes start the same way: you contribute after-tax (non-Roth) dollars to your 401(k), filling the gap between your employee deferral plus employer match and the total §415(c) limit. In 2026 that limit is $72,000 (employee + employer combined; IRC §415(c)) — rising to $80,000 if you are 50 or older with the $8,000 catch-up. Subtract your $24,500 employee deferral (IRC §402(g)) and your employer match, and the remainder is your after-tax headroom. With a small $1,500 match, Priya’s headroom is the full $46,000; a larger match shrinks it dollar-for-dollar. The after-tax money is the raw material; the routes diverge in where it lands.

1. In-plan Roth conversion. You convert the after-tax sub-account into the designated Roth 401(k) sub-account — same plan, same recordkeeper, no new account. The basis converts tax-free; any earnings that accrued before conversion are taxable in the year of conversion.
2. In-service rollover to a Roth IRA. You take an in-service distribution of the after-tax money and roll it to an external Roth IRA. Under IRS Notice 2014-54, you can direct the after-tax basis to the Roth IRA and any pre-tax earnings to a traditional IRA — a clean split that defers tax on the earnings.

Both get your after-tax dollars into a Roth where they grow tax-free forever. The difference is in four trade-offs: investment freedom, the earnings split, future backdoor-Roth risk, and protection & simplicity.

The decision table

Axis 1: investment freedom — the only reason to roll out

The in-plan conversion locks you into your 401(k) menu. If that menu is a typical 20-fund Fidelity or Vanguard lineup with low-cost index options, you lose nothing real — a total-market index fund inside the plan grows identically to one in a Roth IRA. The roll-out earns its complexity only when you want something the plan can’t hold: a specific sector ETF, individual stocks, a target-date fund the plan dropped, or a low-cost share class the plan doesn’t carry.

Check whether your plan offers a self-directed brokerage window (Fidelity BrokerageLink, Schwab PCRA). If it does, you can buy nearly the full market inside the plan after the in-plan conversion — and the investment-freedom argument for rolling out collapses. For Priya, her plan has BrokerageLink, so she can hold her preferred small-cap ETF in the Roth 401(k). The roll-out gives her nothing she can’t already get.

Axis 2: the Notice 2014-54 earnings split

This is the rollout’s genuine technical edge. After-tax money in a 401(k) is two parts: your basis (what you contributed) and earnings (growth before conversion). When you take an in-service distribution, IRS Notice 2014-54 lets you direct the two parts to different destinations: basis to a Roth IRA (tax-free) and earnings to a traditional IRA (deferring the tax on the earnings).

The in-plan conversion can’t do this. You either convert the whole after-tax pool — paying ordinary-income tax on the earnings portion now — or you leave it unconverted. In practice this edge is small if you convert fast (next section), because there are barely any earnings to split. It matters most when earnings have built up — say you contributed after-tax dollars for two years before learning the plan allowed conversion at all.

Axis 3: the trap — how a rollout poisons your backdoor Roth

Here is the part most write-ups bury. Priya, like most high earners over the $150,000–$165,000 single Roth-IRA phase-out, runs a regular backdoor Roth every year: she contributes $7,500 (2026 limit, IRC §219(b)(5)) to a nondeductible traditional IRA and converts it. That move is only fully tax-free if she has $0 of pre-tax money in any traditional IRA on December 31. The pro-rata rule (IRC §408(d)(2)) aggregates all her traditional, SEP, and SIMPLE IRAs and treats every conversion as a proportional mix of pre-tax and after-tax dollars.

Now watch what the rollout’s “clean split” does. If she sends the after-tax basis to a Roth IRA and the earnings to a traditional IRA, she has just created a pre-tax traditional-IRA balance. Even $500 of earnings parked there turns her next backdoor Roth into a partially-taxable event — and it stays taxable every year until she clears the traditional IRA (usually by rolling it into a 401(k), if her plan accepts incoming rollovers).

The in-plan conversion sidesteps all of it. No IRA is created, so there is nothing for the pro-rata rule to aggregate. The backdoor Roth she runs separately stays pristine. For anyone running both strategies — which is most people who can fund a mega-backdoor — this single point usually decides the question in favor of converting in-plan.

Axis 4: protection and effort

• Creditor protection. 401(k) assets get federal ERISA protection — effectively bulletproof from creditors. Roth IRA protection is state-law dependent and generally weaker. If you’re in a litigation-exposed profession, the in-plan route is the safer home.
• Effort and error risk. The in-plan conversion is two clicks and many plans automate it. The rollout means a distribution request, a rollover into the correct accounts, and a 1099-R you must report correctly — every cycle. More steps, more chances to fumble the split and accidentally create the pro-rata problem above.

Worked example: Priya’s $46,000

Priya stuffs $46,000 after-tax across 2026 and uses automatic daily in-plan conversion, so each contribution converts the day it posts. Total earnings before conversion across the year: about $90. She converts $46,000 of basis tax-free plus $90 of earnings (taxable). At her 32% federal marginal rate (single, $197,301–$250,525 for 2026), the tax on the conversion is $90 × 32% = $29. Washington has no state income tax, so that’s the whole bill.

The rollout “saves” her $29 of tax this year by deferring $90 of earnings — and in exchange contaminates a backdoor Roth conversion of $7,500 that would otherwise be 100% tax-free. To run that backdoor cleanly, she’d have to pro-rata it or sweep the $90 back into a 401(k). Trading a $29 saving for that headache is a bad deal. The in-plan conversion wins decisively.

What most people get wrong

• “A Roth IRA is always better than a Roth 401(k).” Not for mega-backdoor money. The Roth 401(k) sub-account grows tax-free identically, often with a brokerage window, and avoids the IRA aggregation that breaks your backdoor Roth. The only durable edge of the Roth IRA is open-architecture investing — and even that closes with a self-directed brokerage window.
• “The split is free money.” Sending earnings to a traditional IRA defers a few dollars of tax but creates a pre-tax IRA balance that triggers the pro-rata rule on future backdoor Roths. The tax deferred is trivial; the damage to a separate strategy is not.
• “I’ll convert at the end of the year.” Every dollar of growth before conversion is taxable when you convert (or, in the split route, becomes a pre-tax IRA balance). Convert as the contribution posts — automatic in-plan conversion is the cleanest version.
• “Once it’s in the Roth 401(k) I’m locked in.” You can roll designated Roth 401(k) funds to a Roth IRA on a later in-service distribution or at separation. The in-plan route preserves the option to roll out; the rollout does not preserve the option to roll back. Choose the route that keeps your options open.

Where the rollout is the right call

The in-service rollover beats the in-plan conversion in a narrow set of cases:

• Your plan allows after-tax contributions and in-service distributions but not in-plan conversion — the rollout is your only route to Roth (check the plan document; see the “plans that support it” guide).
• Your plan menu is genuinely poor (high fees, no index funds, no brokerage window) and you want open-architecture investing.
• You don’t run a backdoor Roth and never will — so the pro-rata risk from a traditional-IRA earnings split is irrelevant to you.
• You’re consolidating accounts at separation anyway and the after-tax money is rolling out with everything else.

Even then, the cleaner move is often to roll the entire after-tax distribution — basis and earnings — to a Roth IRA and pay tax on the small earnings amount now, rather than splitting earnings into a traditional IRA. That keeps your IRA backdoor clean while still getting you the investment freedom you rolled out for.

The decision lever

Ask one question: do you run, or will you run, a regular backdoor Roth? If yes — which is true for nearly everyone who can fund a mega-backdoor — convert in-plan and never let earnings touch a traditional IRA. If no, and your plan menu is weak with no brokerage window, the in-service rollout to a Roth IRA buys you investment freedom worth the extra paperwork. Set up automatic in-plan conversion if your recordkeeper offers it, and you’ve eliminated both the taxable-earnings drift and the pro-rata trap in one switch.