MAGI for IRMAA is simply Adjusted Gross Income (AGI) plus tax-exempt interest. Itemized deductions, including charitable contributions, have nothing to do with it. They do not affect your AGI.
But keep in mind that your IRMAA is based on your MAGI from two, or possibly three, years ago. For example, your IRMAA for 2026 will usually be based on your 2024 tax return, or possibly your 2023 tax return.
On tax returns for 2020 through 2024 the MAGI for IRMAA is Form 1040 line 11 plus line 2a.
Also, in searching the internet keep in mind that there are different definitions of MAGI, depending on what it's being used for. You have to look at the definition that is specifically for IRMAA. MAGI for other purposes would be calculated differently.
