How it works
IRMAA uses your MAGI from two years before the premium year -- 2024 MAGI sets 2026 premiums -- and each of the five tiers above the standard premium applies to your whole MAGI once you're in it, with no partial surcharge at the edge. In the example from CMS's 2026 announcement, a single filer with 2024 MAGI of $136,900 pays the standard-plus-one-tier premium of $284.10 a month; $200 more MAGI, at $137,100, crosses into the next tier at $405.80 a month plus a higher Part D surcharge -- $1,736.40 more for the year from $200 of extra income two years earlier. More on the cliff mechanics and how a December Roth conversion or QCD choice can set a premium two years out in the IRMAA guide.