A Bogleheads thread this summer walked through a familiar setup: married, both 62, one just retired, roughly $2.1 million in traditional 401(k)s, Social Security deferred to 70. The question was whether to start a Roth conversion ladder now or wait. The math says now. The window between retirement and age 73 is the only stretch where this couple controls their own tax bracket.
Once required minimum distributions begin, that control disappears. The IRS decides how much comes out, Social Security stacks on top, and Medicare premiums adjust two years in arrears. A $700,000 conversion spread across seven years, taxed at roughly $160,000 total, is the cheapest tax bill this couple will ever see on that money.
Why the 62-to-72 Window Is the Cheapest Tax Bracket They Will Ever See
With no wages and Social Security deferred, taxable income is close to zero (the quiet stretch between the last paycheck and the first RMD is the whole subject of our free Roth Window guide). The 2026 standard deduction for married filing jointly is $32,200. That means the first $32,200 of any conversion is federally tax-free.
The 2026 married-filing-jointly brackets run 10% up to $24,800, 12% up to $100,800, 22% up to $211,400, and 24% up to $403,550. Converting $100,000 in a year with no other income keeps taxable income inside the 12% bracket. Pushing the conversion higher fills the 22% bracket instead.
The couple’s plan converts $100,000 per year from age 62 through age 68 while both are pre-Medicare, then trims to smaller conversions afterward. Seven years of conversions equal the $700,000 headline. Blended federal tax, layered partly at 12% and partly at 22% once other income appears, works out to roughly $160,000 across the ladder.
Defusing the RMD Tax Bomb
Left alone, that $2.1 million grows. At a 6% return, the balance at age 73 is well over $3.7 million. The first-year RMD lands near $140,000. Add two Social Security checks that started at 70 and taxable income easily clears $220,000, dragging 85% of Social Security into taxable territory and pushing marginal dollars into the 24% bracket.
Then Medicare piles on. In 2026, joint filers with modified AGI above $218,000 pay an $81.20 Part B surcharge per person and a $14.50 Part D surcharge, on top of the $202.90 base Part B premium. Cross $274,000 and the Part B surcharge jumps to $202.90 per person and Part D to $37.50. For a couple, one IRMAA tier can add well over $2,000 a year in premiums, and the lookback is two years, so income at 71 sets premiums at 73.
The conversion ladder shrinks the traditional balance before RMDs are calculated. A $1.4 million balance at 73 produces an RMD closer to $53,000 rather than $140,000. Combined income drops back under the first IRMAA threshold, Social Security taxation stays capped, and the marginal bracket resets to 12% for most of retirement.
When the Trade Does Not Pay Off
Paying tax now to skip tax later only works if the future rate is at least as high. The ladder underperforms if the couple’s retirement bracket ends up lower than today (unlikely with $2.1 million and deferred Social Security), one spouse dies early and the survivor files single at compressed brackets that make conversions more valuable, or Congress cuts rates below current levels. The One, Big, Beautiful Bill made the current brackets permanent, so the 2017-era rate cliff is off the table.
Watch the Medicare line carefully. Age 63 is the first year that matters, because 2026 joint IRMAA kicks in above $218,000 MAGI and premiums lag by two years. Keeping each conversion year’s MAGI under that number preserves the base $202.90 Part B premium.
Three Moves to Run This Week
- Model the RMD you are avoiding. Project your traditional balance to age 73 at 6% growth, divide by 26.5 (the Uniform Lifetime factor at 73), and compare that number to the top of your current 22% bracket. If projected RMD plus Social Security exceeds $211,400, a ladder is on the table.
- Size each conversion to fill a target bracket. Fill the 12% bracket first. Only extend into the 22% bracket if the projected RMD would land in the 24% bracket or higher. Recheck the $218,000 IRMAA line before finalizing each December.
- Pay the conversion tax from a taxable account. Using IRA dollars to cover withholding shrinks the Roth balance and defeats the compounding advantage. With the 10-year Treasury near 5%, short-term Treasuries or a money market are reasonable places to park the tax reserve while the ladder runs.
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