Roth Conversion IRMAA Simulator 2026

How much can you convert to Roth this year before the conversion quietly raises your Medicare bill in 2028? More than most people think, right up until you cross a line you can’t see. This tool finds that line for you.

The decision it solves

A Roth conversion moves money from a traditional IRA to a Roth. You pay ordinary income tax now so the dollars grow tax-free and never face an RMD later. In your 60s, before Social Security and before forced withdrawals, your income is often the lowest it will ever be. That’s the window to convert.

Here’s the catch the spreadsheet misses. Every dollar you convert lands in your modified adjusted gross income, and Medicare reads that number two years later to set your premiums. IRMAA, the income-related monthly adjustment amount, is the surcharge on Parts B and D for higher earners. It isn’t a slope. It’s a set of cliffs. One dollar over a threshold and your premium jumps for the whole year.

So the question isn’t “should I convert.” It’s “how much, before the next dollar costs more than it’s worth.”

How the math works

The simulator stacks your income for the year: pensions, interest, dividends, any Social Security counted, and the conversion you’re considering. It compares that total against two things at once. Your federal bracket, and the 2026 IRMAA thresholds that your 2024 income already locked in for this year (the surcharge always runs on a two-year lookback).

For 2026, a married couple pays the standard Part B premium of $202.90 a month each as long as their MAGI stays at or below $218,000. The first surcharge tier runs from there to $274,000 and adds $81.20 a month per person on Part B, plus $14.50 on Part D. The tool shows you how many dollars of conversion headroom sit between where you are and the next cliff.

A worked example

Take a married couple, both 64, retired, living off taxable savings. Their 2026 income before any conversion is $150,000. They’re deciding how much to convert.

The top of the 22% federal bracket for a couple in 2026 is $211,400 of taxable income. The first IRMAA cliff is $218,000 of MAGI. Convert up to roughly $60,000 and they fill the 22% bracket while staying under the IRMAA line. Their Medicare premiums two years out don’t move.

Push the conversion to $75,000 instead. Now MAGI lands near $225,000, over the $218,000 threshold. That last $7,000 of conversion just triggered the surcharge: about $81 a month more per person on Part B and $14.50 on Part D, for both spouses, for all of 2028. Call it $2,300 in extra premiums. The tax on the conversion didn’t change much. The hidden cost did.

The smart number was the one that filled the bracket and stopped at the cliff. That’s what this tool draws for you.

If your accounts are large

With a seven-figure IRA, one year of “safe” conversions won’t drain it before RMDs hit at 73 or 75. The real plan is a multi-year ladder that may deliberately punch through a lower IRMAA tier some years, because paying a surcharge now beats a decade of RMDs taxed at 32% or higher later. The cliff isn’t always the enemy. Sometimes it’s the toll on the smart road. See Roth conversions without triggering IRMAA for the multi-year view.

Run the number every fall, before December closes the conversion window. The cliff doesn’t move once the year is over.