“How much house can I afford?” is really two different questions. One is how much a lender will approve you for. The other — the one that actually matters — is how much you can comfortably repay without your home turning your budget into a monthly emergency. They are rarely the same number, and the gap between them is where a lot of financial stress is born.
The good news is that affordability comes down to a small amount of math built on three inputs: your income, your existing debts, and your down payment. Once you understand how those pieces fit together, you can set a price ceiling with confidence instead of hope.
The rule lenders actually use: your DTI
Lenders decide how much to lend based mostly on your debt-to-income ratio (DTI) — the share of your gross monthly income that goes to debt payments. They look at two versions:
- Front-end DTI — housing costs alone (principal, interest, taxes, insurance, and any HOA) as a percentage of income.
- Back-end DTI — housing plus every other monthly debt: car loans, student loans, minimum credit-card payments, and so on.
A long-standing guideline is the 28/36 rule: keep housing at or below 28% of gross income, and total debt at or below 36%. Many loan programs stretch the back-end number higher, but the 28/36 line is a sensible ceiling for staying comfortable rather than merely approved.
Being approved for a mortgage is not the same as being able to afford it. The approval is the lender protecting itself — not your budget.
From a monthly payment to a home price
Affordability works backward. Start with the most you can put toward housing each month, subtract the parts that are not loan payment — the property taxes, insurance, and HOA that make up the “TI” in PITI — and what remains is your budget for principal and interest. That figure, combined with today’s interest rate and your loan term, sets the size of the loan you can carry. Add your down payment, and you have your price ceiling.
Three levers move that ceiling:
- Income and debts. Paying off a car loan before you buy can free up hundreds of dollars of monthly borrowing power.
- Interest rate. A one-point change in the rate can swing your affordable price by tens of thousands of dollars.
- Down payment. A larger down payment both shrinks the loan and, past 20%, removes mortgage insurance from the monthly bill.
What the “affordable” number leaves out
The affordability math tells you the maximum a lender’s formula allows. Real life has costs the formula ignores:
- Maintenance. Budget roughly 1% of the home’s value each year for upkeep — more for older homes.
- Closing costs. Typically 2–5% of the price, due up front, on top of the down payment.
- Your other goals. A payment that leaves nothing for retirement, an emergency fund, or a life outside the house is not truly affordable, however the ratio looks.
A useful discipline is to buy below your maximum. The difference between the biggest house you can get approved for and one payment tier below it is often the difference between feeling house-rich and house-poor.
How to use this in practice
- Total your reliable gross monthly income and your existing monthly debt payments.
- Apply the 28/36 rule to find a comfortable housing payment, then work backward to a loan amount at today’s rate.
- Add your planned down payment to get a target price — then shop a tier below it to leave room for the costs the formula skips.