It takes tax filing status and modified adjusted gross income from two years earlier, finds the income-related monthly adjustment amount bracket for Part B and Part D, and shows the surcharge, the household's yearly total and how much income room is left before the next bracket. It is built and publishes here when released.
IRMAA brackets are cliffs: one dollar over a threshold triggers the whole surcharge. That makes the "income room before next bracket" result a powerful planning prompt for Roth conversions and capital gains — and a reason a Medicare client will happily talk to you in the off-season. Life-changing events such as retirement can be reported to Social Security on form SSA-44.