The Mega Backdoor Roth Strategy That Can Add $75,000 to Your 401(k) This Year

A 58-year-old technology executive I heard about on a Bogleheads thread earlier this year already maxed her $24,500 employee deferral by June, added the $8,000 age-50 catch-up on top, and figured she was done for 2026. Her plan document said otherwise. Her employer allows after-tax contributions with an in-plan Roth conversion feature, and that single provision let her push more than $75,000 into the plan this year. The mechanic is Internal Revenue Code Section 415(c), and it is the reason a well-designed corporate 401(k) can absorb roughly three times the standard deferral limit.

What 415(c) Actually Caps

The employee deferral limit gets all the headlines. The 415(c) “annual additions” limit is the real ceiling. For 2026, that combined cap on employee deferrals plus employer contributions plus after-tax contributions is $72,000. Catch-up contributions sit outside that number, so a participant age 50 to 59 can layer in another $8,000, and someone age 60 to 63 can add the SECURE 2.0 super catch-up of $11,250. That produces a hard ceiling of $80,000 at ages 50 to 59 and $83,250 at ages 60 to 63 inside a single plan.

Executives who clear $100,000 in a year typically do it by stacking. Each unrelated employer has its own separate 415(c) limit, so a director who consults through a solo 401(k) on the side can run a second $72,000 bucket alongside the corporate plan. Board fees, K-1 income from a separate LLC, or a spouse’s plan at a different employer all open the door.

The After-Tax Bucket Most People Never Fund

Here is where the money actually gets in. Say a 55-year-old vice president earns $310,000 and receives a 6% employer match. Her deferral plus catch-up is $32,500, her employer adds roughly $18,600, and the two together consume about $51,100. The gap to the 415(c) ceiling, ignoring catch-up, is roughly $20,900 in after-tax contributions she can make from payroll. Those dollars go in after income tax has already been withheld, which is why most participants ignore them.

The value shows up on conversion. If the plan permits either an in-plan Roth rollover or an in-service withdrawal to a Roth IRA, those after-tax dollars move to a Roth wrapper before meaningful earnings accrue. Only the growth between contribution and conversion is taxable, and if the conversion happens the same pay period, that growth is usually a few dollars. That is the mega backdoor Roth, and it is the single largest legal Roth funding channel available to a W-2 employee.

The 2026 Wrinkle High Earners Cannot Ignore

The catch-up itself changed this year. Starting in 2026, employees age 50 and older who earned more than $150,000 in FICA wages in 2025 must direct their catch-up contributions to a Roth 401(k). The old pretax option is gone for this cohort. A 55-year-old in the 24% bracket who used to shave roughly $1,900 off her federal bill through a pretax catch-up now writes that check to the IRS instead. A 62-year-old making the full $11,250 super catch-up loses about $2,700 in current-year deduction.

The trade-off carries an upside. The Roth catch-up compounds tax-free and never triggers a required minimum distribution during the owner’s lifetime, which matters at 73 when a $2 million traditional balance can push ordinary income high enough to make 85% of Social Security taxable and drag the household past the first IRMAA threshold. With the 10-year Treasury near 4.7%, even a modest allocation compounding inside a Roth for 15 years produces a materially larger after-tax pot than the same dollars held in a taxable brokerage.

Three Moves Before Year-End

1. Pull your summary plan description and search for “after-tax contributions” and “in-plan Roth rollover.” If both appear, model your remaining 415(c) headroom: $72,000 minus your projected deferral minus employer contributions equals your after-tax capacity for 2026.
2. Confirm with HR whether after-tax dollars convert automatically each pay period or require a manual request. Automatic conversion is what keeps the taxable growth near zero.
3. If you also earn 1099 or K-1 income, open a solo 401(k) at a low-cost custodian before December 31. That second plan carries its own separate $72,000 annual additions limit and is how executives cross the $100,000 line.

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