On your 2017 tax return, the MAGI for IRMAA is Form 1040 line 37 plus line 8b.
On your 2018 tax return, the MAGI for IRMAA is Form 1040 line 7 plus line 2a.
On your 2019 tax return, the MAGI for IRMAA is Form 1040 line 8b plus line 2a.
Keep in mind that your IRMAA is based on your MAGI from two or three years ago. For example, your IRMAA for 2020 will usually be based on your 2018 tax return, or possibly your 2017 tax return.
