Updated July 2026 with current tax data and rates.
A $1 million IRA at 65 grows past $1.7 million by 73, and then RMDs force roughly $64,000 a year of taxable income on you whether you need it or not. Here’s what took me too long to appreciate: the low-income years between retiring and RMDs are the one window to fix this, and crossing a Medicare IRMAA line by a single dollar raises premiums for you and your spouse for a full year. I built this calculator to model your brackets, RMDs, and IRMAA cliffs year by year, so you can see how much to convert and what it saves over your lifetime.
Why the years before RMDs are the window
Required minimum distributions start at 73 (75 if you were born in 1960 or later). From then on the IRS sets your income floor: about 3.8% of your IRA balance the first year, and a bigger slice every year after. Stack Social Security on top and plenty of retirees land in a higher bracket at 75 than they ever saw while working.
The fix lives in the gap years. After the paycheck stops and before RMDs and Social Security pile up, your taxable income can fall to almost nothing. Those are the years to convert IRA dollars at 10, 12, or 22% that would otherwise come out at 24% or higher later. The usual move is bracket filling: convert just enough each year to reach the top of your target bracket, then stop.
The IRMAA cliffs and the two-year lookback
Medicare premiums are means-tested, and the thresholds are cliffs, not ramps. Cross about $106,000 of income filing single or $212,000 married filing jointly and you pay a surcharge on Part B and Part D premiums. One dollar over the line costs you the full surcharge, and if you’re both on Medicare, you both pay it.
The part that catches people is the lookback: Medicare sets your premiums from your tax return two years back. A conversion at 63 shows up in your premiums at 65. The calculator watches those lines for you and warns you when a conversion year would cross one.
The widow’s penalty
Married couples should run one more scenario. When one spouse dies, the survivor files single: roughly the same income, half the bracket widths, lower IRMAA lines. Money converted while you’re both here and filing jointly is taxed at rates the survivor will never see again. Run the calculator as married, then run it as single, and look at the gap. That gap is the case for converting sooner rather than later.