Quick Answer
Your estimated Roth conversion room before the next IRMAA tier is:
Next IRMAA threshold minus your no-conversion IRMAA MAGI.
For example, if a married couple's no-conversion IRMAA MAGI is $190,000 and the first 2026 joint threshold is $218,000, their rough room before the first surcharge tier is $28,000.
Why Roth Conversions Affect IRMAA
A Roth conversion moves money from a pre-tax retirement account into a Roth account. The converted amount is usually taxable income in the conversion year. Because IRMAA is based on modified adjusted gross income, a conversion can push a household into a higher Medicare premium tier.
This does not automatically mean the conversion is a mistake. Roth conversions can reduce future required minimum distributions, create tax-free Roth growth, and improve estate flexibility. The point is to count the Medicare surcharge before deciding how large the conversion should be.
Step-By-Step Estimate
- Start with income before the Roth conversion: pension, wages, interest, dividends, capital gains, IRA withdrawals, and taxable Social Security.
- Add tax-exempt interest because it is part of IRMAA MAGI.
- Estimate taxable Social Security if benefits are being received.
- Find the next IRMAA threshold for your filing status.
- Subtract your no-conversion MAGI from that threshold.
Example
Suppose a married couple filing jointly has $82,000 of pension income, $64,000 of gross Social Security, $12,000 of qualified dividends and capital gains, and no tax-exempt interest. Depending on how much Social Security becomes taxable, their no-conversion MAGI may sit well below the first 2026 joint IRMAA threshold of $218,000.
If the calculator estimates their no-conversion MAGI at $151,400, their rough room before the first IRMAA tier is $66,600. A $40,000 conversion would likely stay below that tier. An $80,000 conversion would likely cross it.
The Hidden Interaction: Social Security
A conversion can do more than add its own dollars to income. It may also cause more Social Security to become taxable. That means a $20,000 conversion can sometimes increase AGI by more than $20,000 once the Social Security formula reacts.
This is one reason a calculator is better than mental math. The best conversion amount is often near a tax bracket or IRMAA threshold, but the exact room depends on the other income already in the return.
When Crossing IRMAA May Still Be Worth It
Some households intentionally cross an IRMAA tier. That can make sense when future RMDs are expected to be much larger, when one spouse has a shorter planning horizon, when tax rates are unusually low, or when the household wants more Roth assets for heirs.
The smarter question is not "Should I always avoid IRMAA?" It is "Does the long-term benefit of this conversion justify the extra tax and Medicare premium cost?"
Next step: Use the calculator to compare no conversion, a threshold-sized conversion, and a larger intentional conversion.
Calculate conversion room