Mortgage lenders are required to assess your ability to repay the amount you want to borrow. A lot of factors go into that assessment, and the main one is debt-to-income ratio. Your debt-to-income ratio is the percentage of pretax income that goes toward monthly debt payments, including the mortgage, car payments, student loans, minimum credit card payments and child support. Lenders look most favorably on debt-to-income ratios of 36% or less — or a maximum of $1,800 a month on an income of $5,000 a month before taxes.