How to use a mega backdoor Roth 401(k) when your pay is high

How much can a mega backdoor Roth 401(k) take under the $72,000 limit?

For 2026, the after-tax room in a mega backdoor Roth 401(k) is $72,000 minus your $24,500 employee deferral minus everything your employer contributes. With a $9,000 match, that leaves $38,500. The IRS sets the $72,000 total under section 415(c), and your plan can set a lower cap.

Carol (hypothetical, round numbers) is 48 and runs engineering as a VP at a public cloud company. She is a single parent of 2 teenagers and files as head of household. She assumed her pay shut her out. The Roth IRA phase-out for 2026 runs $153,000 to $168,000 for single filers, and the same range for head of household, but it does not apply to in-plan Roth conversions. Two other limits do: the total can't exceed 100% of plan compensation, and many plans cap after-tax contributions at a percentage of pay, so your plan's number can be smaller than the arithmetic says.

Catch-up contributions sit outside the $72,000: $8,000 at age 50 and over, and $11,250 at ages 60 to 63. If your prior-year FICA wages exceeded $150,000, the catch-up must be Roth. Room resets every plan year and never carries forward, so Carol's last December paycheck is her final chance to use it. The calendar below assumes a calendar-year plan and $38,500 of room.

Ask the plan administrator these questions in writing

Carol's problem wasn't the arithmetic. It was that her benefits portal said nothing clear, and she couldn't tell whether her plan converted anything. Email gives you an answer you can point to later, so send these before you change a payroll election.

Watch the source names: the Summary Plan Description often says 'after-tax' in one place and 'Roth' in another, and the two can mean different money. Ask which source your after-tax contributions land in, and whether the plan moves them into the Roth source automatically each payroll or only when you ask. Get both source names exactly as the recordkeeper shows them on a statement.

• HR or recordkeeper: does the plan accept after-tax (non-Roth) contributions, and what percentage cap applies?
• HR or recordkeeper: does it convert to Roth automatically each payroll, or only on request (some plans only quarterly)?
• HR or recordkeeper: does the match have a year-end true-up, and did the plan pass last year's ACP test?
• CPA: how will converted earnings appear in box 2a of Form 1099-R, and should estimated payments rise, since RSU vests are often withheld at the flat supplemental rate and that can fall short?
• Advisor: how does $38,500 a year of after-tax payroll compare with the next 3 years of scheduled stock sales, college bills and gifts?

Carol's $38,500 a year: what starting at age 48, 50 or 53 is worth

Carol defers $24,500 and her employer matches $9,000, so her after-tax room is $72,000 − $24,500 − $9,000 = $38,500 annually. Starting now at age 48 and stopping at age 60 gives 12 years, and 12 × $38,500 = $462,000 moved to Roth. Starting at age 50 gives 10 years, or $385,000. Starting at age 53 gives 7 years, or $269,500. Unused room lapses each December, so waiting 5 years forfeits $192,500 of Roth contributions that can never be made up.

Assuming 5% a year, for illustration, one year's $38,500 grows to about $102,000 over 20 years. That's not a forecast, and every investment can lose money, including what you put in.

The cost lands in her paycheck. $38,500 annually is about $3,200 monthly less take-home pay ($38,500 ÷ 12). Quarterly blackouts stop Carol from selling shares whenever she likes, so her pre-set trading schedule sells newly vested RSU shares on fixed dates to replace it. Those shares carry little gain because the vest set the cost basis, and each sale trims her $3 million position a little.

Before Edward Jones Investments suggests raising her after-tax percentage, it lines up 3 years of the 2 college bills and the $50,000 annual gift against the sale dates. The after-tax slot only gets money her spending playbook doesn't need until after age 60. Giving shares instead of cash for the gift is covered on the donating-stock page.

She gains tax-free growth on money she won't spend for 12 or more years. She gives up flexibility: that $38,500 can't cover a surprise tuition bill, and a blackout stops her from selling extra shares to cover one. She accepts that because college is already funded from scheduled sales. At age 50 she adds the $8,000 catch-up as required Roth, which leaves her after-tax room unchanged.

Why Raj and Lan should keep that cash out of the 401(k)

Raj and Lan (hypothetical, 41 and 39, both engineers at the same chip company) have after-tax room similar to Carol's, but they need $400,000 for a down payment in 3 years. Converted Roth 401(k) money is hard to reach before age 59½. Plans often restrict in-service withdrawals, and earnings in a nonqualified distribution are taxed and can carry a 10% penalty. Their risk capacity is low for a second reason too: 2 paychecks and $1.4 million of stock depend on one employer.

So their stock sales should fill a down-payment fund held in short-term, low-volatility holdings, and they should revisit the after-tax slot after closing. The different answers come from the due date, not from income or risk appetite. Carol's money isn't needed for 12 or more years and her near-term costs are covered. Raj and Lan's biggest bill arrives in 3 years.

Is an in-plan Roth conversion reversible?

A converted amount is permanent, but the contribution rate is not. On most plans you can raise, cut or stop the after-tax percentage any payroll period (it often takes effect within 1–2 paychecks), and you can fill unused room until the last December paycheck. Once a conversion is processed, Roth conversions can't be recharacterized.

If the plan fails ACP testing, the refund usually arrives around March 15 of the following year, and that year's room is lost for good.

What does a January Form 1099-R show if the conversion was slow?

A January Form 1099-R reports the earnings your after-tax money made before it was converted as taxable income in box 2a, so a slow conversion makes that number bigger. Converting every payroll keeps the earnings near zero. Converting only on request or quarterly lets them build.

Carol's first-year Form 1099-R arrives in late January with $2,000 in the taxable box. Her plan converted only on request, and her after-tax money sat in a stock fund for 9 months. At an assumed 35% rate that's about $700 of tax on growth that would have been tax-free. The fix is automatic per-payroll conversion, and checking the source names after the first paycheck: if the recordkeeper still shows the balance under 'after-tax' rather than 'Roth', the conversion never ran.

Front-loading has its own bill. In a plan that matches per paycheck with no true-up, hitting $24,500 by August stops 4 months of match, and $9,000 × 4/12 = $3,000 is gone. Check the true-up before changing percentages.

Then there's the March letter saying the ACP test failed. Highly compensated employees get after-tax money back, the earnings are taxable, and the room can't be reused. Ask for last year's test result before raising the percentage.

Skip or shrink the after-tax slot in these 3 situations

Quick test: put money in the after-tax slot only if your spending playbook won't need it before age 59½, and only after the next 3 years of planned costs (college, gifts, a cash reserve) are covered by cash or scheduled stock sales.

First, the plan accepts after-tax money but lets you neither convert it inside the plan nor withdraw it while still employed. Earnings are then taxed as ordinary income when withdrawn, and a taxable brokerage account in low-turnover index funds can be the better place until you leave. Second, you expect to change jobs soon, since after separation the after-tax basis can roll to a Roth IRA and the earnings to a traditional IRA. Third, you might stop working before age 59½ and need this money, or the ACP test caps highly compensated employees at a few percent of pay. Roth 401(k) earnings need 5 years of Roth participation plus age 59½ to come out tax-free.

This slot also does nothing about concentration itself. Carol's $3 million of employer stock still needs its own selling schedule.

Bring your plan summary and trading schedule to Edward Jones Investments

Talk with Edward Jones Investments before your next payroll election change or before you set the next pre-set trading schedule, because its cooling-off period runs 30 days for most employees and up to 120 days for officers. Bring the Summary Plan Description, a recent pay stub, last year's Form 1099-R if you converted, and the sale dates on your trading schedule. Edward Jones Investments advisors meet clients nationwide by video or phone and can walk through them with you.