# The mega backdoor Roth move most engineers leave on the table

Source: https://www.techinterview.org/post/3233475448/mega-backdoor-roth-engineers-equity/
Updated: 2026-07-02 · techinterview.org

Your 401(k) has two limits, and the space between them holds the largest tax-free account most engineers never open. For 2026 you can defer $24,500 of your own salary into the plan. The IRS caps *total* contributions to that same plan at $72,000. Almost nobody fills the gap, and at AI-lab or senior-FAANG pay it can send tens of thousands of dollars a year into Roth space instead of a taxable brokerage account.

That gap is what the mega backdoor Roth fills. The name is goofy and the mechanics sound exotic, but it mostly comes down to whether your plan has two specific features switched on, and whether you have the cash flow to feed it. The cash flow question is almost always answered by equity.

## The bucket your onboarding deck skipped

A 401(k) holds three kinds of money. The first is your own deferral, pre-tax or Roth, capped at $24,500 in 2026. The second is whatever your employer adds, the match and any profit sharing. The third is a separate **after-tax** bucket, and it is not the same thing as a Roth deferral even though both use already-taxed dollars.

The difference is what happens to the growth. Money in a Roth 401(k) grows tax-free and comes out tax-free. Money sitting in the plain after-tax bucket grows tax-deferred, so the earnings get taxed as ordinary income when you eventually pull them out. That is a worse deal than a Roth and barely better than a brokerage account. The mega backdoor Roth is the act of moving those after-tax dollars into Roth status quickly, so that from that day forward the growth is tax-free.

## Two switches decide whether you can do this at all

Your plan has to accept after-tax (non-Roth) contributions, and it has to let you get them into Roth, either through an in-plan Roth conversion or an in-service withdrawal you roll to a Roth IRA. If either switch is off, the whole thing is dead no matter how much you earn.

Don't assume. Call whoever runs the plan (Fidelity, Schwab, Principal, Vanguard) and ask two things in plain language: does the plan take after-tax contributions beyond my regular deferral, and does it support automatic in-plan Roth conversion. Plenty of large tech employers have both. Plenty of well-funded startups have neither, because the plan was set up in a week by someone who never expected engineers to be maxing it. Some plans also cap after-tax contributions at a percentage of pay, say 10%, which can quietly stop you well short of the full $72,000 even when both features exist, so ask about that ceiling too. The feature you want most is the automatic conversion, for a reason that shows up in a minute.

## The math with the 2026 numbers

Start at $72,000, the total plan limit for 2026. Subtract your $24,500 deferral. Subtract whatever your employer contributes. What's left is your after-tax room.

Say you defer the full $24,500 and your employer adds an $11,500 match. That's $36,000 in the plan, leaving $36,000 of after-tax room you can fill and convert. A smaller match opens a bigger after-tax window, up toward $47,500 if the employer puts in little. If you're 50 or older, catch-up contributions raise your deferral ceiling and the total cap moves with it (to $80,000 in 2026, and $83,250 for ages 60 to 63 where the plan allows the higher catch-up), so the room math shifts but the idea is identical.

To see why this is worth the paperwork: $30,000 a year of after-tax-to-Roth conversions, growing at 7% for twenty years, lands north of $1.2 million that the IRS never touches again. That is the difference between a Roth bucket and a taxable account where every dividend and every sale is a line on your return.

| Route | 2026 ceiling | Income limit | Main catch |
| --- | --- | --- | --- |
| Direct Roth IRA | $7,500 (a little more past 50) | Phases out at high income, so usually closed to you | Most senior engineers earn too much to use it directly |
| Backdoor Roth IRA | $7,500 | None on the conversion | IRA aggregation rule taxes the conversion if you hold other traditional IRAs |
| Mega backdoor Roth (401k after-tax) | Up to about $47,500, depending on your match | None | Needs both plan features on, plus the cash flow to fund it |

## Convert the same week, or the pro-rata rule takes a cut

Here is why automatic conversion matters. The after-tax bucket holds two things: your contributions, which are already taxed, and any earnings on them, which are not. When you convert, your basis crosses over tax-free but the earnings are taxed, pro-rated against the balance. Let the money sit and grow for a year before converting and you hand back a slice of the gain as ordinary income.

The clean version converts every contribution within a day or two of it landing, automatically, so there is essentially no earnings to tax. If your plan only allows manual conversions a few times a year, set a reminder and do it the day after each contribution posts. This is a completely separate rule from the IRA pro-rata mess below, so don't blur the two together.

## The backdoor Roth IRA is the smaller cousin, and it bites differently

Separate move, separate account. A Roth IRA lets you put in $7,500 for 2026 (a bit more past 50), but direct contributions phase out at high income, which rules out most people reading this. The backdoor version gets around that: contribute to a nondeductible traditional IRA, then convert it to Roth. No income limit on the conversion.

The trap is the IRA aggregation rule. The IRS looks at *all* your traditional, SEP, and SIMPLE IRAs together when you convert. If you have a fat rollover IRA from an old job sitting there, your conversion comes out mostly taxable, pro-rated across that balance, even though the fresh contribution was after-tax. The fix is to roll that old IRA *into* your current 401(k) first, if the plan takes incoming rollovers, which empties the IRA side and lets the backdoor conversion come through clean. This is the step people skip right before they get a surprise tax bill.

## Equity is what actually pays for all of this

The after-tax contributions come straight out of your paychecks, so your take-home drops hard during the months you're filling the bucket. For most senior engineers the thing that makes that survivable is RSUs.

RSUs are taxed as ordinary income the moment they vest, at that day's share price, whether you sell or not. Because the vest price becomes your cost basis, selling at or near vest triggers little or no extra tax. The shares you've already been taxed on are the cleanest source of cash you have. Most people at this income level are also far more concentrated in their employer's stock than they would ever choose to be on purpose. Selling vested RSUs you would never buy at today's price, and routing that cash into a diversified Roth, fixes two problems with one trade.

An ESPP works the same way as a funding source. The purchase discount is close to a guaranteed return, and selling the shares (watch the qualifying-versus-disqualifying-disposition timing on the tax) frees more cash to feed the after-tax bucket and the backdoor IRA. The pattern across all of it is identical: sell concentrated equity you would never buy fresh, move the proceeds into tax-free space.

## The order that actually makes sense

Capture the full employer match before anything else, because that is a return you cannot get anywhere else. If you have a high-deductible health plan, an HSA comes next for its rare triple tax break. Then max the $24,500 deferral, choosing Roth or pre-tax on that piece based on whether your bracket today is higher or lower than you expect it to be in retirement. After that comes the after-tax bucket with prompt conversion, then the backdoor Roth IRA, and only then a regular taxable brokerage account. Most engineers stop two rungs too early and leave the after-tax bucket untouched.

## When it isn't worth the trouble

If you're not already maxing the $24,500 deferral, the after-tax bucket is premature, so fill the cheaper, simpler space first. If your plan has no automatic conversion and you know you won't keep up with manual ones, the pro-rata drag and the hassle may not pay for themselves. If you're saving for a house in the next couple of years, remember that Roth earnings aren't freely reachable before 59 and a half, so locking money there can work against a near-term goal. And if you genuinely expect a much lower tax bracket in retirement, a bigger pre-tax balance might beat the Roth, which is a real judgment call rather than a rule.

None of this is tax advice, and the numbers reset every year, so confirm the current limits and your own plan's features against the plan document and a CPA before you move money. The engineers who win at this usually aren't the highest paid ones. They're the ones who called the plan administrator, found out the automatic-conversion box existed, and checked it.