Adjusted Gross Income. Adjusted Gross Income is defined as gross income minus adjustments to income. We suggest you refer to your 2015 federal income tax return to get a quick estimate of your 2016 AGI. On your 2015 return, please refer to: Line 4 if you filed a Form 1040EZ; Line 21 if you filed a Form 1040A; Line 37 if you filed a Form 1040