28/36 Rule Calculator

Traditional mortgage underwriting relies on two primary benchmarks known as the front-end and back-end ratios. The front-end ratio focuses strictly on housing expenses, dictating that your total shelter costs should not exceed 28% of your gross monthly income. Conversely, the back-end ratio examines your total debt obligations—including housing plus other monthly liabilities like student loans, credit cards, and auto payments—recommending that this figure stays below 36%. The mathematical relationships are defined as follows: housing_costs = principal_loan_amount + loan_interest + property_tax + insurance. From there, the front_end_ratio is calculated as housing_costs divided by income. Total monthly debt is found via total_debt = housing_costs + other_debts, and finally, the back_end_ratio is determined by dividing total_debt by income.

Imagine a household earning a gross monthly income of $8,000. They are reviewing a potential new home where the combined monthly housing costs—including principal, loan interest, property taxes, and homeowners insurance—total $2,000. Additionally, they have $600 in other recurring monthly debts, such as an auto loan and student loan payments. First, we calculate the front-end ratio: $2,000 housing costs divided by $8,000 income yields 0.25, or 25%. This falls comfortably under the 28% threshold. Next, we find the total debt by adding housing costs and other debts: $2,000 + $600 = $2,600. To find the back-end ratio, we divide $2,600 by the $8,000 income, resulting in 0.325, or 32.5%. Because both ratios sit below the 28% and 36% targets, this applicant is in a strong financial position to secure loan approval.

To optimize your chances of mortgage approval, always use your gross (pre-tax) income rather than your net take-home pay when running these calculations. Avoid taking on new lines of credit or purchasing major items like a car immediately before applying for a mortgage, as even a small increase in other debts can push your back-end ratio past the critical 36% threshold. Keep in mind that while 28/36 is a classic baseline rule, many modern lenders allow higher maximum ratios depending on your credit score, cash reserves, and down payment size.