Income-Related Monthly Adjustment Amount · Also called: Medicare surcharge, high-income premium
A surcharge added to Medicare Part B and Part D premiums when your MAGI from two years earlier is above a threshold. It is charged per person and rises in steps.
The thresholds are cliffs, not phase-ins: one dollar over a threshold moves you to the next tier's premium for the whole year. For IRMAA, MAGI is AGI plus tax-exempt interest. A married couple on Medicare pays the surcharge twice.
Most tiers are indexed to inflation each year. The Part D surcharge is paid to Medicare even if your drug plan has a zero premium.