Roth Conversions From 63 to 67: Why a Couple’s $500,000 Window Slams Shut at the First RMD

A couple posted on a retirement forum this summer with a question I hear constantly: both spouses just turned 63, they have roughly $1.5 million combined in traditional 401(k)s, and they want to move about $500,000 into Roth accounts before required minimum distributions begin. They assumed they had a full decade to do it, since RMDs don’t start until age 73. The math says otherwise. The genuine conversion window is closer to five years, from 63 to 67, and it slams shut long before the first RMD ever prints on a 1099-R.

Here is why the window is narrower than the calendar suggests, and how to size conversions so the strategy actually lowers lifetime tax rather than accelerating it.

Why the Real Conversion Window Is Only Five Years

Between ages 63 and 66, a couple who has stopped working but hasn’t yet claimed Social Security has an unusual gift: taxable income that they largely control. If pension income is modest and investment income comes mostly from qualified dividends and long-term gains, ordinary income can be pushed almost anywhere the couple wants it.

For a married couple filing jointly in 2025, the 22% bracket runs up to $206,700 of taxable income, and the 24% bracket extends to $394,600. That gives room to convert roughly $100,000 to $125,000 per year and still keep the top dollar taxed at 24% or lower. Four clean years of that pace delivers the $500,000 target.

The window closes for three reasons:

1. Social Security taxation. Once benefits begin at full retirement age (67 for this cohort), up to 85% of those benefits become taxable when combined income crosses modest thresholds. A $60,000 household benefit suddenly adds roughly $51,000 of taxable income before a single dollar of conversion is layered on. The 2027 COLA is tracking near 3.3%, which nudges benefits and thresholds in opposite directions for planning purposes.
2. IRMAA’s two-year lookback. Medicare premiums at 65 and beyond are set from the tax return filed two years earlier. A conversion done at 64 shows up on the Part B and Part D bill at 66. Crossing the first tier can add roughly $70 to $80 per person per month; higher tiers push the surcharge past $400 per person per month. For a couple, that is real money layered on top of the income tax already paid on the conversion.
3. The RMD itself. At 73, the IRS forces distributions from what remains in the traditional account. A $1.5 million balance that grew to roughly $2.2 million by 73 produces a first-year RMD near $83,000, and that number rises every year. Conversions done after RMDs begin must be layered on top of the RMD, which is exactly the tax bracket problem the strategy was supposed to avoid.

Sizing the Conversion Against Today’s Rates

The 24% federal bracket is historically cheap. With the 10-year Treasury near 5%, the opportunity cost of paying conversion tax from a taxable brokerage account is manageable: the after-tax yield given up on the tax payment is roughly 4%, while the Roth grows and distributes tax-free for life and skips RMDs entirely.

A workable plan for this couple: convert about $125,000 in each of the four years from 63 through 66, filling the 22% and lower end of the 24% bracket. Pay the tax from a taxable account so the full converted amount lands in the Roth. Stop or sharply reduce conversions the year Social Security begins, because the marginal cost, once SS taxation and IRMAA are layered in, commonly runs 40% or higher for dollars that looked like 24% dollars on the surface.

That gap between the last paycheck and the first required withdrawal is the cheapest tax environment most people will ever see again, and we sized up how to use it in a free guide to the Roth window.

Three Moves to Make Before Year-End

1. Pull last year’s Form 1040 and calculate exactly how much room remains under the $206,700 top of the 22% bracket for married filing jointly. That number, minus current-year ordinary income, is the ceiling for a comfortable conversion this year.
2. Model the IRMAA impact of any conversion done at 63 or 64 against the Medicare premium bill it will produce at 65 or 66. If the conversion pushes the household past the first IRMAA tier, decide whether the Roth benefit justifies the surcharge or whether to trim the conversion by $10,000 to $20,000 to stay under.
3. Decide on a Social Security start date before finalizing the conversion schedule. Delaying benefits from 67 to 70 extends the low-income window by three additional years and can double the size of the total conversion that fits under the 24% bracket.

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