Mega backdoor Roth: how after-tax 401(k) money becomes Roth

The two limits that create the room

The mega backdoor Roth is not a special provision of the tax code. It is what happens when two ordinary limits are stacked on top of each other and a plan lets you use the space between them.

The first is the elective deferral limit under IRC §402(g): $24,500 for 2026, up from $23,500 in 2025. It covers your pre-tax and Roth 401(k) deferrals combined, and it is a personal limit — it follows you across every 401(k), 403(b), and similar plan you participate in during the year. The second is the annual additions limit under IRC §415(c): $72,000 for 2026, up from $70,000. It caps the total that can be added to your account in a plan year — your deferrals, employer matching and profit-sharing contributions, forfeitures reallocated to you, and after-tax employee contributions (IRS, 2026 limits; Notice 2025-67).

Two details matter. The §415(c) limit is also capped at 100% of your compensation, so it only binds at $72,000 if you earn at least that much from the employer. And unlike §402(g), it generally applies per employer: plans of unrelated employers each get their own $72,000 (IRS, 401(k) and profit-sharing plan limits). That distinction is what makes the solo 401(k) angle below possible.

Where catch-up contributions fit

If you are 50 or older, the 2026 catch-up is $8,000. If you turn 60, 61, 62, or 63 during 2026, SECURE 2.0's higher catch-up applies instead: $11,250 (IRS, catch-up contributions).

Catch-ups do not count against the $72,000. IRC §414(v)(3)(A) provides that catch-up contributions are not subject to the §415(c) limit, and the IRS states the practical result directly: the 2026 overall limit is $72,000, or $80,000 including the age-50 catch-up, or up to $83,250 for those 60 to 63. For mega backdoor purposes, that means a catch-up neither adds to nor subtracts from your after-tax room. You calculate the room on your regular deferrals only, then make the catch-up on top.

How much after-tax room you have

The formula is short:

Here is what that produces for an employee under 50 earning $200,000 who defers the full $24,500, at three levels of employer contribution:

Three things the table hides. First, if you defer less than $24,500, your after-tax room grows by the difference — though for most people a Roth deferral is a better use of the first dollars than an after-tax contribution, because Roth deferrals are Roth from day one with no conversion step. Second, many plans cap after-tax contributions at a percentage of pay well below the legal maximum, so the plan's number may be smaller than the table's. Third, profit-sharing contributions are often decided and deposited after year-end; if you fill the limit to the dollar in December and the employer then adds more, the plan will have to correct the excess. A margin is cheap insurance.

The three plan features you need

None of this is a taxpayer right. Your plan document has to provide for it, and it takes three features working together:

1. After-tax (non-Roth) employee contributions. These are not Roth deferrals. They go in with money you have already paid tax on, they create basis, and their earnings are pre-tax. A plan that offers only pre-tax and Roth deferrals has no mega backdoor.
2. A way out of the after-tax bucket: in-plan Roth conversion or in-service withdrawal. Either the plan lets you convert the after-tax account into its designated Roth account — the mechanics are covered in our in-plan Roth conversion guide — or it lets you withdraw the after-tax account while still employed and roll it to a Roth IRA. Many plans let you take after-tax money out at any time, because the statutory restrictions on elective deferrals do not apply to it, but that is a plan choice; see in-service withdrawals for how plans typically draw those lines.
3. No meaningful frequency limit. A plan that allows one conversion or withdrawal a year technically supports the strategy, but every month the money sits in the after-tax account it earns taxable growth. The best versions allow conversions at any time, and some recordkeepers automate them with each paycheck.

The fastest way to find out is your summary plan description, then one email to the plan administrator asking all three questions at once.

Why so many plans don't offer it

Most employers that skip after-tax contributions are not making a judgment about Roth money. They are avoiding a test. Under IRC §401(m), matching contributions and after-tax employee contributions must pass the actual contribution percentage (ACP) test, which compares the average contribution rate of highly compensated employees with that of everyone else (IRS, common qualified plan requirements).

Safe harbor plan designs can exempt the employer match from that test. They cannot exempt after-tax contributions: after-tax employee contributions are always ACP-tested. And after-tax money is used almost entirely by higher earners, who are the only people with cash flow left after maxing deferrals. In a plan where a handful of executives contribute $30,000 or more after-tax and rank-and-file employees contribute nothing, the test fails. The correction is to refund excess contributions to the highly compensated employees, which can undo the strategy after the fact.

So the plans that offer it tend to be large employers with broad participation, or plans where after-tax contributions are low enough across the board to pass. If your employer has an HCE-heavy workforce, the absence of the feature is usually a testing problem, not an oversight — and lobbying HR rarely changes the math. If you are a highly compensated employee in a plan that does offer it, know that a refund of part of your after-tax contribution after year-end is possible.

Why speed matters

The contribution itself is already taxed, so converting it costs nothing. The earnings are another matter. Earnings on after-tax contributions are pre-tax money: convert them and they are ordinary income; leave them and they grow tax-deferred, not tax-free. The longer contributions sit in the after-tax account before conversion, the more taxable earnings pile up and the worse the end result.

Converted within days, the taxable portion is typically trivial. Left for a year in an equity fund, it can be a meaningful fraction of the contribution. That is why frequency matters more than any other plan feature — and why automatic in-plan conversion, where available, is worth turning on.

In-plan conversion vs. rollover to a Roth IRA

Both routes end with Roth money. They differ in what happens to the earnings and where the money lives afterward.

In-plan conversion. The after-tax account, generally with its earnings, moves to the plan's designated Roth account. The contribution portion is tax-free; any earnings converted with it are taxable that year and reported on Form 1099-R. Everything stays inside the plan, keeps ERISA creditor protection, and uses the plan's investment menu. The full mechanics — reporting codes, the five-year clocks, the absence of withholding — are on the in-plan conversion page and are not repeated here.

In-service withdrawal and rollover. If the plan lets you withdraw the after-tax account, IRS Notice 2014-54 lets you split a single distribution by tax character. You can direct the after-tax basis to a Roth IRA and the pre-tax earnings to a traditional IRA in simultaneous direct rollovers, so the earnings are not taxed now (IRS, rollovers of after-tax contributions). The notice does not let you take only the after-tax dollars; each distribution still carries a proportional share of pre-tax and after-tax amounts. What it allows is routing those shares to different destinations.

The IRA route gives you a wider investment menu and avoids current tax on earnings. The in-plan route is simpler, often more frequent, and keeps plan-level creditor protection. One trade-off is easy to miss: pre-tax earnings rolled to a traditional IRA become part of the IRA balance the pro-rata rule counts if you also do a backdoor Roth IRA. If you use both strategies, that small traditional IRA balance can make your IRA backdoor partly taxable.

The solo 401(k) version

Self-employed people with no employees other than a spouse can, in principle, write the mega backdoor into their own plan. The ACP problem largely disappears: with no non-highly compensated employees in the plan, there is no lower-paid group to fail against. The plan still needs the same three features, and that is where it gets harder. Many off-the-shelf solo 401(k) documents from large brokerages do not permit after-tax contributions or in-plan conversions; you may need a provider that offers a customized plan document, usually for an annual fee. We compare the account types in solo 401(k) vs. self-directed IRA.

The math also differs. A sole proprietor's employer contribution is limited by net self-employment earnings, so lower earners may run into the 100%-of-compensation cap before the $72,000. And because §415(c) generally applies per employer, someone with a W-2 job at an unrelated company and a side business may have a separate $72,000 in each plan, while the $24,500 deferral limit is shared across both. That combination is powerful and easy to get wrong; it is worth having a CPA or the plan provider confirm how your plans are counted.

Mega backdoor vs. backdoor Roth IRA

The names are similar; the mechanics are not. The backdoor Roth IRA is a nondeductible traditional IRA contribution — $7,500 for 2026 — converted to a Roth IRA, and it is complicated by the pro-rata rule if you hold other pre-tax IRA money. The mega backdoor runs entirely through your 401(k), can move several times as much each year, and is unaffected by your IRA balances. They are not alternatives; a high earner with access to both can do both in the same year. For where these fit among other high-earner moves, see the advanced strategies guide.

Is it still allowed? Yes. As of October 2026, no enacted law has eliminated after-tax contributions or their conversion. A 2021 House proposal would have barred conversions of after-tax plan money; it did not become law. That history is a reason to use the room while it exists rather than to assume it is permanent.

How to set it up, step by step

1. Confirm the three features — after-tax contributions, in-plan conversion or in-service withdrawal of after-tax money, and how often either is allowed. Ask about any plan cap on after-tax contributions in the same message.
2. Max your regular deferrals first, pre-tax or Roth. Decide that split separately — see pre-tax or Roth.
3. Estimate employer contributions for the full year, including any year-end profit share, and compute your after-tax room with a margin.
4. Elect the after-tax percentage through payroll, spread across the year or front-loaded if cash flow allows. If your plan's match is calculated per pay period, check that front-loading deferrals doesn't cost you match later in the year.
5. Turn on automatic conversion if offered; otherwise convert or roll over as often as the plan permits.
6. Keep records of after-tax basis and every conversion, and check the 1099-R the following January.

Before committing cash flow to this, it helps to see what an extra $30,000 to $47,000 of Roth money a year is worth over a career compared with a taxable account. The Roth calculator lets you model the long-run difference on your own assumptions.