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.
There are about a dozen different definitions of MAGI, depending on what it's being used for. The preceding information is specifically for the Income-Related Monthly Adjustment Amount (IRMAA) for Medicare premiums. MAGI for other purposes would be calculated differently.
I thought that in addition to the tax-exempt interests, the 15% of my SS retirement that is tax free needed to be added back to the AGI for the calculation of the IRMAA? Please confirm.
MAGI for IRMAA is as I stated above. Nothing gets added back except tax-exempt interest. See 2501. What is Modified Adjusted Gross Income (MAGI)? on the Social Security Administration web site.
MAGI for other purposes is calculated differently.