Your MAGI (modified adjusted gross income) is your AGI plus some deductions that are added back. The deductions you must add back depend on the government benefit in question.
For many tax payers, MAGI is the same as AGI.
However, for some federal tax breaks, your eligibility is based on your MAGI, not your AGI.
IRA Related
For example, MAGI determines your eligibility to contribute to a Roth IRA or whether you are allowed to deduct a contribution to a traditional IRA. To calculate your MAGI for all matters related to IRA, take your AGI and add back any deductions you have taken for student loan interest, a contribution to a traditional IRA, foreign earned income and housing exclusions, savings bond interest and employer adoption benefits.
Medicare Related
For Medicare, your Part B and Part D premiums are based on your MAGI, in this case, you need to add back any tax-exempt interest income to your AGI.
Health Insurance Related
If you purchase health insurance through the federal Affordable Care Act (ACA) marketplace, your MAGI is used to determine if you quality for a reduced premium. The ACA MAGI adds back any deducted foreign income, nontaxable Social Security benefits and tax-exempt interest.