To calculate your debt-to-income ratio, add up all of your monthly debts – rent or mortgage payments, student loans, personal loans, auto loans, credit card payments, child support, alimony, etc. – and divide the sum by your monthly income. For example, if your monthly debt equals $2,500 and your gross monthly income is $7,000, your DTI ratio is about 36 percent. (2,500/7,000=0.357).