The term 28/36 rule is a guideline used by underwriters and lenders use to see if you can afford the home you want to buy. In general, this rule is considered one of the best ways to calculate the amount of mortgage payment debt, you can afford based on your income. Many lenders require that potential homebuyers' maximum household expense-to-income ratio is 28%, with a maximum total debt-to-income ratio of 36% in order to be approved for a mortgage.