The IRMAA adjustment is calculated by the Social Security Administration. They normally use your tax return from two years earlier. So your 2024 IRMAA would be based on your 2022 tax return. Sometimes the tax return information from two years earlier is not available when the IRMAA has to be calculated. In that event they would use your tax return from three years earlier. See the following article on the Social Security Administration web site for more information.
Description of the Medicare Income-Related Monthly Adjustment Amount Determination Process
