MAGI for increased Medicare premiums ("IRMAA") is simply AGI plus tax-exempt interest. But your IRMAA is based on your MAGI from two or three years ago. For example, your IRMAA for 2023 will usually be based on your 2021 tax return, or possibly your 2020 tax return. On your 2020, 2021, or 2022 tax return, the MAGI for IRMAA is Form 1040 line 11 plus line 2a.
