Your Modified Adjusted Gross Income (MAGI) is used to determine how much, if any amount, you are able to contribute to a Roth IRA. The calculation for your MAGI begins with your Adjusted Gross Income (AGI). From there, some modifications are made to your MAGI. Below are the most common modifications. For a step-by-step guide to calculating your MAGI, use Publication 590-A, Worksheet 2-1. Start with Your AGI: Find your Adjusted Gross Income (AGI). This can be found on IRS Form 1040, Page 2, Line ...