To calculate your debttoincome ratio, you add up all your monthly debt payments and divide them into your gross monthly income. To determine your DTI, first add up all of your monthly debt payments (use minimum payments for revolving credit such as credit cards).All you have to do is add up all your regular monthly bills plus an estimated car and insurance payment. Three-family properties: Maximum 50 percent debt ratio with 75 percent of rent included in income. Home inspection is mandatory and three-family buyers must. Your debttoincome ratio is a percentage that compares your monthly debt payments to your gross monthly income. Debt-to-income (DTI) ratio is the percentage of your monthly gross income that is used to pay your monthly debt and determines your borrowing risk. The front-end DTI determines the impact your new mortgage payment will have on your monthly income. A "good" credit score is considered anything starting in the mid-600s and up. DTI: Debt-to-income ratio.