Quick Read
A $1.6M 401(k) growing 7% annually reaches $4.4M by RMD age, pushing the first $166,000 withdrawal into a near-40% effective tax rate.
Converting $200,000 pre-tax to Roth at 32% costs $64,000 upfront but eliminates roughly $122,000 in future RMD taxes, netting $58,000 in present-value savings.
Pay the conversion tax from outside taxable savings, not from the 401(k) balance itself. Using retirement funds to cover the bill erases most of the benefit.
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A 58-year-old software executive earning $400,000 posted on a personal finance forum last month asking why her advisor kept pushing an “in-plan Roth conversion” on her $1.6 million traditional 401(k). She already maxes contributions, already funds a backdoor Roth IRA, and is a decade from retirement. The advisor’s pitch: pay tax now at her current marginal rate to eliminate a specific future problem. The math worked out to roughly $58,000 in taxes she would never have to pay on required minimum distributions.
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That number comes from the interaction of three rules that most high earners underestimate.
Why the RMD Cliff Hits Highest Earners Hardest
Traditional 401(k) balances grow tax-deferred, and every dollar comes out as ordinary income after age 73. On a $1.6 million balance growing at 7% for 15 years, the account reaches roughly $4.4 million by RMD age. The first-year RMD alone is around $166,000.
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Stacked on top of Social Security, a pension, and taxable investment income, that RMD lands squarely in the 32% bracket that begins at $201,775 for singles in 2026. It also makes 85% of Social Security benefits taxable and blows through every IRMAA tier. A retiree in the 32% federal bracket who also triggers IRMAA and full Social Security taxation faces an effective marginal rate near 40 cents on the dollar for every incremental RMD dollar.
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What an In-Plan Roth Conversion Does
The in-plan Roth conversion is a single-plan transaction. Pre-tax 401(k) dollars are reclassified as Roth 401(k) dollars inside the same account. The converted amount is added to that year’s ordinary income, taxed at current marginal rates, and then grows tax-free. Since Roth 401(k)s no longer require distributions during the owner’s lifetime under SECURE 2.0, that money never generates an RMD.
Roughly 96% of 401(k) plans offered a Roth component in 2024, and most now permit in-plan conversions. Availability requires only a call to HR.
The $58,000 Math, Line by Line
Take the executive above. Her taxable income sits comfortably in the 32% bracket, which runs from $201,775 to $256,225 for singles in 2026. She converts $50,000 per year from traditional to Roth for four years, totaling $200,000.
Tax paid up front: $200,000 converted at 32% equals $64,000 in federal tax spread across four years.
Growth in the Roth bucket: That $200,000 compounds at 7% for roughly 17 years until age 75, reaching about $632,000. Every dollar comes out tax-free and never triggers an RMD.
Tax that would have been owed: Had the $200,000 stayed pre-tax, it would have grown to the same $632,000 and been drawn down through RMDs at a blended effective rate near 40% once Social Security taxation and IRMAA are layered in. Present-value the tax stream across a 20-year withdrawal window at a 4% discount rate and the figure lands near $122,000.
Net savings: Roughly $58,000 in present-value tax, before counting the IRMAA relief in each of those retirement years.
Savings compound if conversions happen in the gap year between W-2 income ending and Social Security starting, when marginal rates often drop to 22% or 24%. This is the classic Roth conversion window, and 24/7 Wall St. has written a full report on how to sequence it (see The Roth Window).
Two Traps to Sidestep
The conversion is irrevocable. Recharacterization was eliminated by the 2017 tax law and stayed dead under the One Big Beautiful Bill. A conversion that pushes taxable income above $256,225 for singles lands the top slice in the 35% bracket, so size each year’s amount to fill the 32% bracket without spilling over.
High earners now face a mandatory Roth catch-up rule that took effect January 1, 2026, for anyone who earned more than $150,000 in FICA wages the prior year. That already pushes $8,000 of catch-up contributions ($11,250 for ages 60 to 63) into Roth. Layer the in-plan conversion on top rather than treating it as a substitute.
What to Do This Quarter
Call the 401(k) administrator and confirm a Roth 401(k) source and in-plan conversion permission. If both exist, request the conversion form.
Model the conversion amount that fills the current bracket without crossing into the next one. For a single filer, that ceiling sits at $256,225 in 2026; for joint filers, $512,450.
Pay the conversion tax from taxable savings, not from the converted balance. Using 401(k) dollars to cover the bill shrinks the tax-free base and erases most of the benefit.
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