The mega backdoor Roth in a solo 401(k) — why your plan document probably says no

The "mega backdoor Roth" gets breathless coverage as a loophole. It isn't a loophole — it's three ordinary plan features used in sequence. What actually gates it, in my experience, is never the tax code. It's the plan document, which most people have never read.

The three contribution types

A 401(k) can hold three different kinds of participant money, and the distinction matters enormously here:

1. Pre-tax deferrals — the normal kind, capped at $24,500 [VERIFY: 2026 402(g) limit].
2. Roth deferrals — same cap, shared with pre-tax.
3. After-tax contributions — a separate, older, less common bucket. These are not Roth. You get no deduction going in, and the earnings grow pre-tax and are taxable on the way out.

That third bucket is what makes the mega backdoor possible. The deferral limit of $24,500 [VERIFY] only applies to buckets 1 and 2. The real ceiling on everything combined — deferrals, employer contributions, and after-tax — is the annual additions limit: $72,000 [VERIFY: 2026 415(c) limit], plus catch-up if you're 50 or older.

The mechanics, in order

Say you're 45, running a business as an S-corp, paying yourself a $150,000 W-2 salary:

1. You defer the maximum $24,500 [VERIFY] (pre-tax or Roth, your choice).
2. Your company makes an employer contribution — say 25% of wages, $37,500.
3. That leaves $72,000 − $24,500 − $37,500 = $10,000 [VERIFY: arithmetic against current limits] of unused annual-additions room, which you fill with after-tax contributions.
4. You promptly execute an in-plan Roth conversion of the after-tax money (or, if the document allows in-service distributions of after-tax amounts, roll it to a Roth IRA).

Since you already paid tax on the after-tax contribution, converting it costs nothing extra — only the earnings that accrued between contribution and conversion are taxable. Convert monthly or immediately and that number rounds to zero.

Notice something about that example, though: with a healthy employer contribution, the leftover room was only $10,000. The mega backdoor is biggest when you skip or reduce the employer contribution and fill the whole gap with after-tax dollars instead — trading a deduction today for Roth treatment forever. Whether that trade makes sense is a real planning question that depends on your bracket now versus later, not a default.

The S-corp wage trap

For S-corp owners there's a second ceiling that surprises people: after-tax contributions (combined with everything else) can't exceed 100% of your compensation — and for you, compensation means W-2 wages, not distributions.

The popular S-corp tax play — pay yourself the smallest defensible salary to minimize payroll tax — works directly against the mega backdoor. At a $50,000 salary, your total annual additions are capped at $50,000, full stop. If maximizing Roth accumulation matters to you, the "right" S-corp salary is a joint optimization between payroll tax and retirement-plan room, and most owners have only ever solved half of it.

Why your plan document probably says no

Here's the practitioner reality: the big free brokerage solo 401(k)s are built on prototype documents designed for simplicity and low support cost. Historically, most of them did not permit after-tax contributions, and several still don't permit in-plan Roth conversions [VERIFY: current feature set of major providers — this changes year to year].

To run the mega backdoor you need a document that affirmatively provides:

• After-tax (non-Roth) employee contributions — the entire strategy rests on this;
• In-plan Roth conversions or in-service distributions of after-tax amounts — the exit door;
• And ideally, separate accounting for the after-tax source, so the earnings tracking is clean.

If your current provider's document lacks these, the fix is a non-prototype / custom plan document from a document provider or TPA, typically a few hundred dollars a year [VERIFY: typical pricing]. You keep the same investments; you're paying for paperwork that says yes instead of paperwork that says no. For someone shoveling $30,000+ a year into Roth, that fee is noise.

Two compliance notes that come with the upgrade:

• After-tax contributions in a plan with employees trigger ACP testing, which they will almost certainly fail if only the owner uses them. This article is about solo plans — the moment you have eligible employees, the mega backdoor stops being a casual strategy and needs real design work.
• A solo 401(k) with more than $250,000 [VERIFY: threshold] in assets files Form 5500-EZ annually — and aggressive contribution strategies get you there fast.

Don't let the earnings sit

The after-tax bucket's earnings are pre-tax. Every month the money sits unconverted, the eventual conversion picks up a taxable component and the recordkeeping gets messier. The clean pattern is mechanical: contribute after-tax, convert within days, repeat. Some custom documents even support automatic conversion sweeps [VERIFY: availability]. Set it up once and stop making decisions.

Who this is actually for

The honest audience for the mega backdoor is someone who is already maxing their deferral, already has cash flow beyond the employer contribution, and wants more tax-advantaged space. That's a minority of business owners — but if you're in it, this is among the highest-leverage moves available, and the barrier is almost always a $0 plan document that was never designed to say yes.

Read your adoption agreement. If the after-tax box isn't checked — or doesn't exist — that's the project.