IRMAA stands for Income-Related Monthly Adjustment Amount - a surcharge Medicare adds to your Part B and Part D premiums when your modified adjusted gross income (MAGI) crosses certain thresholds. Roth conversions add to MAGI, so an aggressive conversion can quietly push you over an IRMAA tier and raise your Medicare premiums two years later.

When IRMAA-aware is on with the fill-to-bracket strategy, the simulation caps each year's conversion so MAGI stays under the first IRMAA threshold (currently $106,000 single / $212,000 married filing jointly). Each year it converts the smaller of:

• the room remaining in your selected federal tax bracket, and
• the room remaining below the first IRMAA threshold (threshold minus your MAGI before the conversion).

So if you have $40,000 of room left in the 22% bracket but only $20,000 of room below the IRMAA threshold, the conversion is clamped at $20,000. When the option is off, the conversion fills to the top of the bracket regardless of where MAGI lands.

Caveats:

• The cap only watches the first IRMAA tier. If you're already past tier 1, the option won't shield you from crossing tier 2.
• The threshold values are not inflation-adjusted in this cap, though Medicare's published IRMAA brackets do grow over time.
• MAGI here is AGI plus tax-exempt interest, where AGI includes only the taxable portion of Social Security - not the gross benefit.
• The cap applies at every age, but IRMAA surcharges only matter once Medicare is active.