Your 401(k) can probably accept far more money than you’re putting into it, and for high earners, that unused room can mean tens of thousands of untapped, tax-efficient retirement savings a year.
The reason is that there are two separate 401(k) limits, and most people only encounter one. The standard paycheck deferral is capped at $24,500 for 2026. That’s the number your payroll system enforces and the number on your contribution election screen. However, your 401(k) account can receive up to $72,000 a year, counting every dollar from every source. Your employer’s contributions take up part of that space. For many high-income earners, most of the rest goes unused.
The mega backdoor Roth is a mechanism for filling that gap, and for filling it with Roth money specifically. Money that grows and eventually comes out untaxed. Let’s explore how that works.
The Mega Backdoor Roth Strategy Explained
The mega backdoor Roth isn’t a product you buy, and it isn’t a loophole. It’s a pair of provisions your employer either wrote into your 401(k) plan document or didn’t, funded by payroll deductions like every other contribution you make. For someone with real room, that’s roughly $40,000 a year over twenty years. At a 7% return, that could become about $1.6 million. You’d have contributed $800,000. The other $840,000 is tax-free growth.
The Third Bucket
Most people experience their 401(k) as a single choice. Pre-tax or Roth. You picked one at enrollment, or your employer picked for you, and you haven’t thought about it since.
Those two options are more alike than they look. Both are elective deferrals, meaning money you choose to divert out of your paycheck before it reaches you, and both draw from the same $24,500. Put the full amount into pre-tax, and you have nothing left for a Roth. Split it evenly, and you’ve used the same room. Choosing a Roth changes when you pay the tax. It doesn’t change how much you can contribute.
A third option is often available, yet the retirement plans that offer it rarely advertise it. It’s called an after-tax contribution, though your benefits portal might label it “voluntary after-tax” or “non-Roth after-tax” depending on who built the software. This third bucket isn’t an elective deferral. The $24,500 limit doesn’t apply to it at all. The only ceiling is the $72,000.
Roth vs. After-Tax 401(k)
This is where nearly everyone gets lost. After-tax and Roth sound like the same thing, and in one respect they are. Both are funded with money that’s already been taxed. The difference is what happens to the growth. Roth growth is never taxed. Growth on an after-tax 401(k) contribution gets taxed as ordinary income when you withdraw it, exactly like your pre-tax balance.
That makes an after-tax contribution to a 401(k) a mediocre deal. You paid tax on the way in, and you’ll pay ordinary income tax on the earnings on the way out. Compare that to the same money in a regular brokerage account, where growth you’ve held longer than a year comes out at long-term capital gains rates, and where anything you still own at death passes to your heirs with the unrealized gain wiped clean. For a top-bracket earner, that’s 37% versus 23.8%, and the brokerage account gets the step-up at death.
The after-tax 401(k) does shelter your growth from year-to-year taxation, which a brokerage account doesn’t. That’s worth something. It just isn’t worth enough to overcome an exit at ordinary income rates.
That’s why the mega backdoor Roth has a second step. To implement a mega backdoor Roth, you make an after-tax 401(k) contribution, but only so you have a place to put money, not a place to leave it. The second step involves converting that money into a Roth.
How to Do a Mega Backdoor Roth
Step 1: Find Your Gap
The arithmetic is one line. Start at $72,000, subtract what you defer from your paycheck, and subtract everything your employer puts in on your behalf. What’s left is the room available for after-tax contributions.
The deferral is what you control and already know. Note that it counts the same whether you chose pre-tax or Roth, since both are elective deferrals drawing on the same $24,500. The after-tax gap is the one people forget. It includes the match, any profit sharing, and any safe harbor contribution, all of it counted at the gross amount regardless of whether you’re vested yet.
That gap is also where the surprise lives.
| Elena $300,000 salary 3% employer match | Marcus $300,000 salary 10% employer contribution | |
|---|---|---|
| Deferral | $24,500 | $24,500 |
| Employer contribution | $9,000 | $30,000 |
| Counted so far | $33,500 | $54,500 |
| After-tax gap | $72,000 – $33,500 = $38,500 | $72,000 – $54,500 = $17,500 |
Marcus gets $21,000 more from his employer than Elena does. Elena has a $21,000 larger after-tax gap than Marcus. Those are the same $21,000, and the relationship is exact, not approximate. Every dollar your employer contributes is a dollar you can’t.
Nobody should turn down a match over this. Marcus is better off. But the two of them are filling the same ceiling with different kinds of money, and that’s worth seeing clearly. Marcus’s ceiling fills up with his employer’s pre-tax dollars, which will be taxed when he withdraws them. Elena’s fills up with her own after-tax dollars, which are headed into a Roth account. The better the benefit package, the smaller the Roth opportunity sitting behind it.
One adjustment if you’re 50 or older. Catch-up contributions sit outside the $72,000 ceiling rather than inside it, so at 50 or older your real ceiling is $80,000, and $83,250 if you’re between 60 and 63. Your deferral limit rises too, to $32,500 at age 50 and $35,750 from age 60 to 63. Run the same math with those numbers.
Now you have a figure. The next question is what happens to the money you put in that space, because it doesn’t become Roth money on its own.
Step 2: Implementing the Conversion: Why the Money Can’t Stay There
An after-tax 401(k) contribution lands in its own subaccount inside your 401(k), separate from your pre-tax and Roth money. Two things live in there. Your contributions, which have already been taxed, and whatever those contributions have earned, which hasn’t.
Converting sorts them out. The contributions move into Roth treatment with no tax due, because you already paid it. The earnings are taxed as ordinary income in the year you convert. And you can’t take one without the other. A distribution from that subaccount comes out proportionally, so there’s no way to convert your contributions and leave the earnings behind to deal with later.
That means the interval between contributing and converting is the only variable that matters.
Example
Elena puts $38,500 into her after-tax subaccount over the year. If her plan converts every payroll, her earnings never have time to accumulate, and she converts almost nothing taxable. If she waits and converts once in December, she’s earned roughly $1,300 along the way, and at her marginal rate that’s a tax bill under $500. Annoying, not damaging.
Leave it longer, and the arithmetic turns. That same $38,500, sitting unconverted for five years, throws off about $13,600 in earnings. Converting then costs her around $5,000 in tax. Nothing went wrong. She just let taxable earnings pile up inside an account designed to hand them off.
So, the mechanism has a preferred setting, and it’s a plan feature rather than a decision you make each year. Some plans run an automatic in-plan Roth conversion, sweeping after-tax money into your Roth 401(k) every pay period without you doing anything. That’s the version you want. Others let you request an in-plan conversion manually, which works fine if you remember to do it. And some don’t offer in-plan conversions at all, but do allow an in-service distribution, meaning you can pull the after-tax money out while still employed and roll it into a Roth IRA yourself.
The destination differs slightly. Money converted in the plan stays in the plan, with your plan’s investment menu and rules. Money rolled out to a Roth IRA is yours to invest however you want. Both end up as Roth money. The mechanical difference that matters more is frequency, because if your plan permits only one in-service distribution a year, that forces you into the December version whether you like it or not.
Does Your Plan Support the Mega Backdoor Roth Strategy?
Two provisions must be in place for the mega backdoor Roth, and they’re independent of each other. Your plan must permit after-tax contributions and either in-plan Roth conversions or in-service distributions. Either one alone is a dead end. A plan that accepts after-tax money but won’t let you convert it leaves you making mediocre contributions to an account that taxes your growth at ordinary income rates. A plan with generous conversion provisions and no after-tax option gives you nothing to convert.
The fastest way to find out is often your own benefits portal. If your contribution election screen offers a third option next to pre-tax and Roth, something labeled “after-tax” or “voluntary after-tax,” you have half the answer already.
For the rest, ask. Your plan’s summary description covers this, and it’s the document you actually have access to rather than the full plan document itself. If reading forty pages sounds worse than sending an email, two questions get you there.
- Does the plan allow after-tax contributions that aren’t Roth deferrals?
- Does your plan allow in-plan Roth conversions or in-service distributions of after-tax money?
Ask both questions. Asking only the first is how people end up contributing into a trap.
Be prepared for the answer to take a round trip. HR often doesn’t know, and the person who does is at the recordkeeper. That’s normal, and it isn’t a sign that you’re asking about something exotic.
Why Plans Say No
Plenty of plans will say no, and the reason usually has nothing to do with you. After-tax contributions are subject to a nondiscrimination test that compares what highly compensated employees put in against what everyone else does. Because the people who use this feature are overwhelmingly high earners, the test often fails, and when it does, the plan must refund money to the very people who contributed it.
Some employers avoid the feature entirely rather than manage that. Others allow it but cap after-tax contributions at a percentage of pay well below your actual room. If your plan caps you at 10% of salary, that’s your number regardless of what your own arithmetic showed.
What if They Say Yes?
If the answer is yes, the implementation is easier than the explanation. You set an after-tax contribution percentage in the same screen where you set your deferral, and it comes out of your paycheck every period like everything else. This is not a check you write in December.
The natural moment to turn it on is the one most high earners already experience without thinking about it. If you defer aggressively, you hit the $24,500 limit somewhere in the middle of the year. Your deferrals stop, your paycheck grows, and the increase shows up as ordinary take-home pay you weren’t budgeting for. That’s the moment your after-tax contributions can pick up where your deferrals left off.
Some plans do that for you automatically through a spillover feature that redirects contributions into the after-tax bucket the instant you hit the deferral ceiling. If yours has that feature, turn it on once, and you’re done. If it doesn’t, you’ll need to change your election yourself when the crossover happens, which is the one date worth putting on a calendar.
The crossover may not fund the whole gap on its own. If you’d been deferring pre-tax, that income becomes taxable the moment the deferrals stop, so your take-home rises by less than the amount you were deferring. However, if you’d been deferring to a Roth, the swap is closer to one-for-one, since that money had already been taxed. Either way, what gets freed up isn’t necessarily as much as your remaining room requires. Some people run a smaller after-tax election across all twelve months instead. In that case, nothing requires you to wait for the crossover.
Most plans have controls that stop you before you breach the ceiling, so an election that turns out to be slightly too aggressive gets caught rather than penalized. This is worth rechecking each January, since the $72,000 ceiling moves with inflation and your employer’s contributions may change. But the machinery runs itself, and if your plan converts automatically, it runs itself all the way into Roth treatment.
Is a Mega Backdoor Roth Worth It?
Yes. In our view, it’s worth it. If you are a high-income earner, you will likely need to save more for your retirement than the standard 401(k) contribution limits allow. Investing after-tax money in a Roth is a tax-efficient way to do that, and it’s relatively easy. There are other options, though. We encourage you to reach out if you would like to discuss whether this is the right move for you.
Disclosure: Golden Road Advisors LLC (“Golden Road Advisors”) is a registered investment advisor. Advisory services are only offered to clients or prospective clients where Golden Road Advisors and its representatives are properly licensed or exempt from licensure.
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