Short answer: For IRMAA, focus on Medicare MAGI. ACA MAGI is commonly used for health insurance marketplace subsidy planning before Medicare.
Why this gets confusing
People often learn MAGI while shopping for health insurance marketplace coverage, then later see MAGI again when Medicare premiums begin. The words are similar, but the planning question changes.
Medicare MAGI for IRMAA
For IRMAA planning, a practical working formula is adjusted gross income plus tax-exempt interest. That is why municipal bond interest, Roth conversions, taxable IRA withdrawals, and capital gains can matter.
ACA MAGI before Medicare
ACA MAGI is used for Affordable Care Act health insurance marketplace subsidy estimates. It can be important before Medicare eligibility, but this site's calculator is built for Medicare premium surcharges, not ACA subsidy estimates.
Education sponsor placement for Medicare transition planning or retirement income education resources.
Planning handoff
If you are approaching Medicare, shift from marketplace subsidy thinking to Medicare premium thinking. Age 63 can matter because of the two-year lookback, and your first Medicare premium year may depend on income before you enroll.
Next step: use the IRMAA planning checklist before a Roth conversion, IRA withdrawal, capital gain, or home sale.