Should a 63-Year-Old Couple With $1.5 Million Convert to a Roth Before Medicare? Here’s the Math
Most retirees think the tax window before RMDs is wide open, but a hidden Medicare surcharge closes it years earlier than expected, and the timing of a Roth conversion in your early 60s can haunt your premiums at 65.
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A married couple, both 63, sitting on roughly $1.5 million in traditional 401(k)s has a narrow window to reshape their tax bill for the next 30 years. That window closes faster than most retirees realize, because Medicare’s income-related surcharges use a two-year lookback. Conversions you do at 63 show up on your first Part B premium at 65.
A Roth conversion usually helps a couple this size. The real question is how much to convert in 2026 and 2027 before the IRMAA meter starts running, and whether to keep going after it does. That gap between the last paycheck and the first RMD is the cheapest tax rate most retirees will ever see again, which is why we built a free guide to using the Roth window before it closes.
Why the Window Is Smaller Than It Looks
Assume the couple retired this year, is living on cash and taxable brokerage assets, and has not yet claimed Social Security. Their ordinary income is close to zero. On a joint return in 2026, the standard deduction is $32,200, and the 22% bracket runs to $211,400 of taxable income, with 24% kicking in above that and 32% above $403,550.
That means they can pull roughly $243,000 out of the traditional 401(k) as a Roth conversion and still finish the year in the 22% bracket. Federal tax on that conversion lands near $37,000, a blended rate around 15%. Compare that to their likely bracket at 75, when RMDs, Social Security, and any pension stack on top of each other and easily push a $2 million-plus traditional balance into the 24% or 32% band.
IRMAA Trap They Cannot See Yet
Here is what the bracket math misses. Medicare premiums at 65 will be set from the 2026 tax return. For a joint filer in 2026, Part B stays at the base premium as long as modified adjusted gross income is at or below $218,000. Cross that line and the first surcharge tier hits; cross $274,000 and it steps up again, with parallel tiers on Part D adding $14.50, $37.50, $60.40, $83.30, or $91.00 per person per month depending on the band.
For this couple, the tightest constraint in 2026 is the $218,000 MAGI ceiling, which binds before the 24% bracket at $211,400 of taxable income. A conversion sized to fill the 22% bracket sits just under both. Push to $250,000 of conversion to grab a slice of the 24% bracket and you volunteer the couple for roughly $185 per month in combined Part B and Part D surcharges each, starting at 65. That is close to $4,400 over the first year of Medicare for two people, on top of the extra federal tax.
Why Doing It Anyway Often Wins
The counterweight is the future RMD. A 63-year-old today falls under SECURE 2.0’s RMD age of 75. Twelve years of compounding on $1.5 million at even a modest return produces a balance where the first RMD alone can exceed $100,000, layered on top of two Social Security checks that will already be growing. The 2027 COLA is tracking near 3.3%, and benefits keep compounding.
The opportunity cost of the tax paid today is real. With the 10-year Treasury at 5%, its highest reading in a year, paying $37,000 in federal tax from a taxable account means giving up a locked-in yield on that cash. Even so, once you model a joint return at 75 with Social Security taxed at the 85% inclusion cap, RMDs, and permanent IRMAA exposure, most couples in this bracket clear a break-even inside 10 years.
What to Do Before Year-End
- Size the 2026 conversion to two ceilings, not one. Target taxable income at or just below $211,400 and MAGI at or just below $218,000. That typically means converting in the $180,000 to $200,000 range, depending on interest, dividends, and any capital gains already booked.
- Delay Social Security to 70 if cash flow allows. Every year benefits stay off the return is another year to convert without the conversion piling on top of taxable Social Security, and the delayed credits raise the base benefit by about 8% per year.
- Model the second IRMAA tier carefully. If the projected RMD at 75 pushes the couple into the 24% bracket regardless, accepting the first surcharge tier at 65 and 66 to convert more aggressively at 24% now can still beat paying 24% or 32% on forced distributions later. Run the numbers at both ceilings before December.
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Jake has been been working in financial media for almost 15 years. He focuses on all things personal finance for 24/7 Wall St. with high hopes to educate and entertain. Most recently, Jake spent 12 years working various roles at The Motley Fool. He started copy editing fool.com content, worked on premium and marketing campaigns, and helped launch The Ascent, a personal finance brand.
His work has been featured on platforms like MSN, Yahoo Finance, USA Today, and more. He's written about credit cards, social security, ETFs, savings accounts, and just about anything else you can imagine when thinking about money. Jake love to cook, play golf, and tell people he's never had a cavity. (It's true!)
