A dual-income couple earning $300,000 between them, ages 60 and 58, has roughly $1.6 million in pre-tax 401(k) accounts and plans to stop working at 62. Their actual question, the one that gets asked on Bogleheads threads every week, concerns what happens to that pre-tax balance between the day the paychecks stop and the day required minimum distributions begin at 75.
That 13-year window is the most valuable tax-planning real estate this couple will ever own. Used well, it is worth roughly $145,000 in federal tax savings over the first eight years alone.
Why the Gap Years Change the Math
While working, this couple sits squarely in the 24% federal bracket, which runs from $211,400 to $403,550 of taxable income for married couples filing jointly in 2026. Converting traditional 401(k) dollars to a Roth now means paying 24 cents on every dollar moved, plus state tax. There is no arbitrage.
The moment they retire at 62, the picture inverts. With Social Security delayed to 70 and no RMDs until 75, their only taxable income is whatever spins off a taxable brokerage account, plus any small pension. Call it $50,000 a year. They drop from the 24% bracket to the 12% bracket overnight, and the top of the 22% bracket sits at $206,700 of taxable income.
The Bracket-Filling Conversion
Stack the $32,200 standard deduction on top of the 22% threshold and the couple has roughly $238,900 of gross income they can absorb each year before any dollar gets taxed at 24%. Subtract the $50,000 of other income and that leaves about $188,000 of headroom to convert annually at a blended rate of roughly 18%.
Round the conversion to $150,000 a year for a margin of safety. Over eight years that moves $1.2 million out of the traditional 401(k) and into a Roth, at a federal tax cost of roughly $27,000 per year, or $216,000 total.
Now run the alternative. Leaving the traditional balance untouched, it compounds at a 7% return assumption against a 4.5% 10-year Treasury baseline. RMDs layered on top of two Social Security checks push the couple back into a higher bracket, with 85% of Social Security becoming taxable and an effective marginal rate closer to 32% once IRMAA Medicare surcharges hit on the two-year lookback. Stretched across the RMD years, lifetime federal tax lands near $360,000 versus $216,000 paid up front. The eight-year capture is the $145,000 figure.
The SECURE 2.0 Wrinkle Before They Retire
One catch applies in the final working years. Because both spouses earned more than $150,000 in 2025, any catch-up contributions in 2026 and beyond must go to a Roth 401(k), not pre-tax. The standard cap is $24,500, with a $8,000 catch-up for those 50 and older and an enhanced $11,250 super catch-up for ages 60 to 63. That removes a small pre-tax shelter today but lines up with the conversion strategy: every Roth dollar accumulated now is one fewer dollar that needs to be converted later.
What to Actually Do
- Map the gap. Identify every year between the last paycheck and age 75, and project taxable income in each. The conversion target is whatever fills the 22% bracket without spilling into the 24% bracket. Recalculate every November once dividends and capital gains are known.
- Pay the conversion tax from a taxable account, never from the converted balance. Withholding from the 401(k) shrinks the Roth and triggers a 10% penalty on anyone under 59½.
- Stop converting two years before Medicare enrollment if the projected MAGI crosses an IRMAA threshold. The lookback turns a single oversized conversion into surcharges of $70 to $400 per person per month for a full year. A fee-only CPA who runs a multi-year tax projection earns the fee on this decision alone.
The bracket-filling window closes the day Social Security or RMDs begin. The couple that uses it captures a permanent tax discount on money they already own. The couple that does not pays the IRS twice: once at 24% on the way in, and again at a higher effective rate on the way out.
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