The mega backdoor Roth

The 'mega backdoor Roth' is a strategy available in some 401(k) plans that lets you contribute additional after-tax money above the standard pre-tax limit and convert it to Roth. Done right, it can move $30,000 or more per year into Roth status, on top of regular contributions. It's the most powerful legal retirement savings strategy for high earners with the right kind of 401(k).

The total 401(k) limit

Most people know the 'employee contribution' limit ($23,500 in 2026). Fewer know there's a TOTAL 401(k) limit — employee + employer + after-tax contributions combined — of about $70,000 in 2026. The gap between your personal contribution + the match and $70k is the space the mega backdoor fills.

How it works

1. Contribute the regular $23,500 to your 401(k) in pre-tax or Roth dollars.
2. Check if your plan allows after-tax (non-Roth) contributions. Not all plans do — this is the gate.
3. If yes, contribute after-tax dollars up to the total combined limit.
4. Immediately convert (or transfer) those after-tax contributions to Roth — either via in-plan Roth conversion or an in-service distribution to an external Roth IRA.
5. The contribution amount converts tax-free; any small earnings during the brief holding period get taxed as ordinary income.

Who this is for

High earners who have already maxed every other tax-advantaged account (401(k), HSA, Roth or backdoor Roth IRA) and still have savings capacity. If you're saving 30% of a large income and need more places to put it, this strategy can double or triple your annual Roth-space contributions.

The full stack, with 2026 numbers

A worked year: what it's actually worth

Take Lena, 38, earning $200,000 with a 5% match ($10,000). She defers the full $23,500, leaving roughly $36,500 of after-tax space under the ~$70,000 cap. Her plan allows after-tax contributions with automatic daily in-plan Roth conversion, so she sets a 19% after-tax payroll percentage and fills the space across the year. Because each contribution converts within days, her taxable earnings on conversion round to nearly zero — the whole $36,500 lands in Roth at essentially no tax cost beyond the income tax she already paid on her salary.

Compound that: $36,500 a year for 20 years at 7% grows to roughly $1.6 million, every dollar withdrawable tax-free in retirement. Had the same money gone to a taxable brokerage instead, dividend taxes along the way plus capital gains on withdrawal would cost an estimated $200,000–$350,000 of that ending value depending on her brackets. The strategy's entire edge is that wrapper — same salary, same investments, radically different tax destination.

Execution, step by step

1. 1
   Interrogate the plan

   Ask your 401(k) administrator two exact questions: does the plan accept after-tax (non-Roth) contributions, and does it allow in-plan Roth conversions or in-service distributions? Both yes = green light.

2. 2
   Compute your space

   Total limit (~$70,000 for 2026) minus your deferral minus projected employer contributions. Leave a small buffer if your match is uncertain or paid late.

3. 3
   Set the payroll percentage

   Divide your space by remaining pay periods and set the after-tax percentage. Confirm your regular deferral still maxes first — some plans sequence contributions oddly.

4. 4
   Convert immediately and automatically

   Turn on automatic conversion if offered; otherwise calendar a monthly manual conversion. Every week after-tax money sits unconverted, it accrues earnings that will be taxed at conversion.

5. 5
   Verify at tax time

   Expect a 1099-R for the conversion showing a small taxable amount (the earnings). Check that the plan tracked your after-tax basis correctly — this is the number that keeps the conversion nearly tax-free.

Common failure modes

• The match-crowding miscalculation: contributing so much after-tax that a year-end employer true-up match gets squeezed against the total limit. Leave room for every employer dollar.
• The lazy conversion: after-tax money left unconverted for months generates taxable earnings, converting the free strategy into a mildly taxed one and creating basis-tracking headaches.
• Highly-compensated-employee refunds: plans that fail nondiscrimination testing on after-tax contributions refund them to high earners. Ask whether the plan has failed the ACP test before committing to a big percentage.
• Doing it before the foundation: this is the LAST tax-advantaged dollar, after the full deferral, HSA, and regular backdoor Roth. Skipping cheaper shelters to fund this one is out of order.

In-plan conversion vs. rolling out to a Roth IRA

Plans offer one or both exits for the after-tax money, and the choice matters a little. In-plan Roth conversion is the low-friction default: money moves the same day, often automatically, and stays inside the 401(k) with its strong creditor protection. The in-service rollover route sends after-tax contributions to your own Roth IRA instead, buying you unlimited fund choice, no plan fees, and more flexible withdrawal ordering later — at the cost of paperwork every time and, in some plans, a limit on how often you can do it. Either route wins by a mile over not doing the strategy; if your plan offers both, in-plan automation usually beats out-of-plan optimization simply because the version that runs without your attention is the version that actually happens all twelve months.

The bottom line

If your plan supports it, the mega backdoor Roth roughly triples the Roth space available to a high earner — commonly $30,000–$40,000 a year on top of everything else, at near-zero tax cost when conversions run automatically. The whole game is two plan features and a payroll percentage. Make the phone call; the answer is worth more than most people's entire year of investment tinkering.