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

A 58-year-old technology executive discussed on a Bogleheads forum earlier this year had already maxed her $24,500 employee deferral by June, stacked the $8,000 age-50 catch-up on top, and assumed she was finished contributing for 2026. Her plan document said otherwise. Her employer allows after-tax contributions paired with an in-plan Roth conversion feature, and that single provision let her push more than $75,000 into the plan this year. The governing rule is Internal Revenue Code Section 415(c), which is why a well-designed corporate 401(k) can absorb roughly three times the standard deferral limit.

What 415(c) Actually Caps

The employee deferral limit captures most of the attention, but the 415(c) “annual additions” limit is the real ceiling. For 2026, that combined cap covering employee deferrals, employer contributions, and after-tax contributions is $72,000 per plan. The IRS also applies a separate compensation ceiling of $360,000 for 2026, meaning employer match calculations cannot use pay above that threshold. Catch-up contributions sit entirely outside the $72,000 figure, so a participant age 50 to 59 can layer in another $8,000 on top, and someone age 60 to 63 can add the SECURE 2.0 super catch-up of $11,250. The result is a hard ceiling of $80,000 for ages 50 to 59 and $83,250 for ages 60 to 63 inside a single plan.

Executives who clear $100,000 in annual contributions typically do it through stacking. Each unrelated employer carries its own separate 415(c) limit, so a director who also consults through a solo 401(k) 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 that door.

The After-Tax Bucket Most People Never Fund

Here is where the money actually flows. Consider a 55-year-old vice president earning $310,000 who receives a 6% employer match. Her deferral plus catch-up totals $32,500, her employer adds roughly $18,600, and together those two figures consume about $51,100 of the $72,000 415(c) ceiling. The remaining gap of roughly $20,900 represents after-tax contributions she can make directly from payroll. Because those dollars enter the plan after income tax has already been withheld, most participants simply ignore them.

The value appears on conversion. When a plan permits either an in-plan Roth rollover or an in-service withdrawal to a Roth IRA, those after-tax dollars move into a Roth wrapper before meaningful earnings have a chance to accumulate. Only the growth between contribution and conversion is taxable, and when the conversion happens within the same pay period, that growth is typically just a few dollars. That mechanic is the mega backdoor Roth, and it remains the single largest legal Roth funding channel available to a W-2 employee.

The 2026 Wrinkle High Earners Cannot Ignore

The catch-up rules themselves changed materially this year. After issuing final regulations in September 2025, the IRS put the Roth catch-up mandate into full effect on January 1, 2026. Employees age 50 and older who earned more than $150,000 in FICA wages from their employer in 2025 must now direct all 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 once trimmed roughly $1,900 from her federal tax bill through a pretax catch-up now sends that money to the IRS upfront. A 62-year-old making the full $11,250 super catch-up loses roughly $2,700 in current-year deduction. Note that this threshold is measured per employer, not across multiple jobs, so workers with FICA wages split among unrelated employers may fall below the limit at each one individually.

The trade-off carries a genuine long-run upside. Roth balances compound tax-free and never trigger a required minimum distribution during the owner’s lifetime. That distinction matters at age 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 currently yielding around 4.7%, even a modest allocation compounding inside a Roth over 15 years can produce a materially larger after-tax outcome than the same dollars sitting in a taxable brokerage account.

Three Moves Before Year-End

1. Pull your summary plan description and search for the phrases “after-tax contributions” and “in-plan Roth rollover.” If both appear, calculate 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 on those contributions near zero.
3. If you also earn 1099 or K-1 income from an unrelated source, 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 it is how some executives push total annual contributions past $100,000.

Editor’s note: This update corrects the Roth catch-up wage threshold to $150,000 in 2025 FICA wages (the IRS raised the base figure from $145,000 in November 2025), adds that the IRS issued final regulations implementing the mandate on September 16, 2025, notes the IRS compensation ceiling of $360,000 for 2026, and updates the 10-year Treasury yield to the current reading of approximately 4.7%.

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