MAGI for IRMAA is simply Adjusted Gross Income (AGI) plus tax-exempt interest. But your IRMAA is based on your MAGI from two or three years ago. For example, your IRMAA for 2024 will usually be based on your 2022 tax return, or possibly your 2021 tax return. On your 2021, 2022, or 2023 tax return, the MAGI for IRMAA is Form 1040 line 11 plus line 2a. See
SSA - POMS: HI 01101.010 - Modified Adjusted Gross Income (MAGI) - 01/05/2024
Yes they look back a couple years to figure the IRMAA for the next year.
https://secure.ssa.gov/apps10/poms.nsf/lnx/0601101031
