IRMAA is based on your MAGI from two or three years ago. MAGI for IRMAA is simply Adjusted Gross Income (AGI) plus tax-exempt interest. For example, your IRMAA for 2026 will usually be based on your 2024 tax return. See
SSA - POMS: HI 01101.010 - Modified Adjusted Gross Income (MAGI) - 01/05/2024
If your income changes use form SSA-44 to adjust the IRMAA
https://www.ssa.gov/forms/ssa-44.pdf
