The bank's number isn't your number
A lender will tell you the maximum you qualify to borrow. That number is built around what you can technically carry without defaulting — not around what leaves room for savings, travel, an emergency fund, or simply not feeling stretched every month. Plenty of buyers make the mistake of treating the approval ceiling as a target instead of a limit.
The gap between "what I qualify for" and "what I should actually spend" is usually where buyer's remorse comes from a year or two after closing — not from the house itself, but from a payment that quietly crowds out everything else in the budget.
The two ratios doing the real work
Underwriting comes down to two debt-to-income ratios. The front-end ratio looks at your housing payment alone against your gross income. The back-end ratio adds in every other recurring debt — car payments, student loans, credit cards, other mortgages — and measures that total against income.
A common comfortable target is 28% front-end and 36% back-end, though many conventional programs allow back-end ratios well above that with strong credit or extra reserves. The number a lender approves and the number that actually feels comfortable to live with are informed by the same math, but they don't have to be the same answer.
What your down payment actually changes
A bigger down payment lowers your loan amount, which lowers principal and interest — and on a conventional loan, crossing 20% down removes PMI entirely. But it does nothing to your property tax or insurance bill, which scale with the home's value, not your loan balance. A common mistake is assuming a larger down payment makes an otherwise-too-expensive home affordable, when tax and insurance alone can still stretch the budget.
The costs that aren't the mortgage payment
Property tax, homeowners insurance, HOA dues if the property has them, and ordinary maintenance all sit on top of principal and interest — and none of them show up in a simple "loan amount × rate" estimate. A realistic affordability number has to include all of it, which is exactly why a full PITI (principal, interest, taxes, insurance) estimate matters more than a bare loan-payment figure.
Run your own numbers
The math above is straightforward once you have your actual income, debts, and target down payment in front of you — you don't need to take a lender's word for it before you even start looking. The House Affordability and Debt-to-Income calculators below use the same ratios described here, built to let you test different scenarios before you're sitting across from an underwriter.