If you're a software engineer in Sorrento Valley, a research scientist in Torrey Pines, or a physician in La Jolla, you've probably already done the obvious things. You max the 401(k). You fund an HSA if you have one. You've heard about the backdoor Roth IRA and maybe you do that too. And then you look at what's left over each year and think, that's it? That's the whole tax-advantaged menu?
For some people, it isn't. There's a fourth door in certain employer plans, and most people who could walk through it don't know it exists. It goes by the unofficial name "mega backdoor Roth," which sounds like marketing but is actually a fairly boring combination of two ordinary plan features.
What the mega backdoor Roth actually is
Start with a distinction most people never need to make. Your 401(k) has two separate limits, not one.
The first is the employee deferral limit, the number everyone knows. For 2026 the IRS set that at $24,500, up from $23,500 in 2025, with an additional catch-up amount available at age 50 and above. That's the cap on what you personally choose to defer out of your paycheck, whether pre-tax or Roth.
The second is the total annual additions limit under Section 415(c), which covers everything that lands in your account from all sources. For 2026 that figure is $72,000. Your deferrals count toward it. So does every dollar of employer match, profit sharing, or non-elective contribution.
Subtract the first from the second and you get the gap. If you defer $24,500 and your employer puts in $12,000, that's $36,500 of the $72,000 accounted for, leaving roughly $35,500 of unused space. The mega backdoor Roth is simply the process of filling part of that gap with after-tax contributions, then converting them to Roth so future growth comes out tax-free in retirement.
The conversion piece is the part that matters. After-tax money sitting in a 401(k) is not especially interesting on its own, because the earnings on it grow tax-deferred and get taxed as ordinary income later. The value shows up when those dollars move into a Roth bucket, either through an in-plan Roth rollover or a distribution to a Roth IRA. The IRS covers the mechanics of both in its guidance on rollovers of after-tax contributions in retirement plans. The speed of that conversion matters too, which is a point we'll come back to.
Two things your plan has to allow
Here's where a lot of enthusiasm meets reality. This isn't a tax election you make on your return. It's a plan design feature, and plenty of plans simply don't have it.
Your plan document has to permit after-tax contributions, which are a distinct contribution type and not the same thing as Roth deferrals. Many plans allow Roth deferrals and no after-tax contributions at all. Separately, the plan has to give you a way to get those after-tax dollars into Roth treatment, either through an in-plan Roth conversion feature or through in-service distributions that let you roll them out while you're still employed.
If either piece is missing, there's no strategy to run. The place to confirm this is your summary plan description or a direct question to your benefits team, and it's worth asking about both features by name rather than asking generally about "the mega backdoor Roth," since plan administrators often don't recognize the nickname.
Who this actually helps
The honest answer is a narrower group than the internet suggests. This tends to be worth evaluating when someone is already maxing the standard employee deferral, already funding a Roth IRA or a backdoor Roth IRA where income limits apply, and still has meaningful cash flow left over that would otherwise go into a regular brokerage account.
That last condition is the one people skip past. After-tax 401(k) contributions come out of take-home pay with no deduction. If funding them means you're not building cash reserves, not covering a tax bill from vesting equity, or leaning on a credit line in a slow month, the tax benefit isn't compensating you for the liquidity you gave up. Money in a 401(k) is genuinely hard to reach before 59½ without planning around it, which is a tradeoff we walk through in more detail in our post on bridge accounts and liquidity planning for high earners.
There's also a sequencing question. San Diego households with concentrated equity compensation often have several things competing for the same dollars in the same quarter, and the mega backdoor Roth is one item on a list that includes diversification decisions, estimated tax payments, and college funding. Where it ranks depends on the household. That coordination problem is a large part of why we built the San Diego HENRY strategy as a standalone framework rather than treating high earners as a generic planning case. If the term itself is new to you, our short explainer on what a HENRY is, San Diego edition, covers the basics.
Where people get tripped up
A few misunderstandings come up often enough to be worth naming, and each one is worth confirming with your CPA or plan administrator before you act on it.
The most common is assuming the $72,000 figure is your personal after-tax allowance. It isn't. It's the total for the account, and your employer's contributions eat into it. Two people at the same salary with different match formulas have different amounts of room, and the person with the more generous match has less.
The second is letting after-tax money sit unconverted. Any earnings that accumulate on after-tax contributions before conversion are taxable when you convert them. Plans that offer automatic or daily conversion make this a non-issue. Plans that require you to submit a manual request each time mean the timing is on you, and a delay of several months in a strong market can create a tax bill nobody expected.
The third is nondiscrimination testing. After-tax contributions are subject to ACP testing in many plans, and if highly compensated employees contribute at rates the broader workforce doesn't match, some of those contributions can be refunded after year-end. A refund isn't a disaster, but it's disruptive, and in a plan with a lot of high earners it's a real possibility worth asking about.
The fourth is treating this as automatically better than a taxable brokerage account. It usually isn't a close call for someone with a long horizon and no near-term need for the money. It gets closer for someone who wants flexibility, who may leave the country, or who expects a low-income window in the future where converting other assets makes more sense. Those are conversations for you, your CPA, and your advisor together, not a rule of thumb.
A sensible way to think about the decision
Start with what your plan allows, because that answer takes five minutes and makes the rest of the analysis relevant or irrelevant. Then look at your actual gap for the year, using your real match rather than an assumed one. Then evaluate whether the dollars you'd be committing are genuinely surplus after cash reserves, tax obligations, and anything you're saving toward in the next few years.
If all three line up, this is one of the larger pieces of tax-advantaged space available to a high earner, and it tends to be underused simply because it's buried in plan paperwork nobody reads. If they don't line up, that's useful to know too, and it saves you from optimizing something that wasn't your constraint in the first place.
If you'd like to walk through your own plan document and where this fits alongside your equity compensation and tax picture, you can schedule a complimentary review with BAS Financial.
Frequently Asked Questions
Is the mega backdoor Roth the same as a backdoor Roth IRA?
No. A backdoor Roth IRA involves contributing to a traditional IRA and converting it to a Roth IRA, and the amounts involved are capped at the annual IRA contribution limit. The mega backdoor Roth happens inside an employer retirement plan, uses after-tax 401(k) contributions, and can involve substantially larger amounts. They're separate strategies, and some people are eligible for both.
How do I find out whether my plan allows this?
Your summary plan description is the authoritative source. Look for whether the plan permits after-tax employee contributions as a distinct category, and separately whether it permits in-plan Roth rollovers or in-service distributions. If the document isn't clear, your benefits team or plan administrator can confirm both features by name.
What happens if I contribute after-tax dollars but never convert them?
The contributions themselves keep their after-tax character and aren't taxed again on withdrawal. The earnings on them, though, grow tax-deferred and are taxed as ordinary income when distributed. That's why plans offering automatic conversion are meaningfully easier to work with, and why the timing of manual conversions is worth confirming with your plan administrator.
Do the 2026 limits change what's possible compared with prior years?
The limits rise modestly most years, and 2026 followed that pattern, with the employee deferral limit moving to $24,500 and the total annual additions limit to $72,000. The mechanics didn't change. A slightly higher total limit means slightly more room, but the deciding factors are still whether your plan allows the feature and how much your employer contributes.
Does this make sense if I'm also holding a large concentrated stock position?
It depends on how those two things interact, and it's worth evaluating with an advisor and CPA rather than in isolation. Committing cash to an illiquid retirement account while carrying concentration risk in a taxable account is a sequencing question, not a yes-or-no one, and the right order varies by household.
This material is intended for general public use. By providing this content, Park Avenue Securities LLC and your financial representative are not undertaking to provide investment advice or make a recommendation for a specific individual or situation, or to otherwise act in a fiduciary capacity. Eligibility for Mega Backdoor Roth strategies varies by employer plan and individual circumstances. Tax laws are subject to change. Consult your tax advisor, CPA, attorney, and financial professional before implementing any strategy.