Here's a link to the definition of Modified Adjusted Gross Income (MAGI) on the pennie web site. The basic definition on that page is "MAGI is adjusted gross income (AGI) plus the following, if applicable: untaxed foreign income, non-taxable Social Security benefits, and tax-exempt interest."
On your Form 1040, AGI is line 11. Tax-exempt interest is line 2a. Non-taxable Social Security benefits is line 6a minus line 6b. Untaxed foreign income might be a little more complicated. It does not appear on Form 1040. Post details if you have any foreign income.
