How Much House Can You Afford? The 28/36 Rule, Honestly Applied

The rule, in one paragraph

The conventional US guideline has two halves, and both matter. The front-end ratio says total housing costs should stay at or below 28% of gross monthly income. The back-end ratio says all debt payments together — housing plus car loans, student loans and credit-card minimums — should stay at or below 36%. Whichever limit binds first is your answer. Lenders will often approve more; the 28/36 pair is what keeps a mortgage from crowding out everything else you spend money on.

The half that gets misquoted is the first one. Housing costs means principal and interest plus property tax, homeowners insurance, HOA dues where they apply, and private mortgage insurance when the down payment is under 20%. Miss those and you will size a loan against a budget that does not exist.

Worked example: $9,000 a month, two very different answers

Take a household with $9,000 of gross monthly income and $600 of existing debt payments, shopping a hypothetical 6.5% rate over 30 years.

Our affordability calculator applies the 36% test: 36% of $9,000 is $3,240, minus $600 of debts leaves a $2,640 monthly payment, which amortizes a loan of $417,676.56. That is a real and useful ceiling — but every dollar of it is spent on principal and interest, with nothing left for tax or insurance.

Now run the 28% test properly. The housing ceiling is 28% of $9,000 = $2,520. Suppose your quotes come in at $400 a month of property tax and $120 of homeowners insurance — $520 of escrow. That leaves $2,000 for principal and interest, and $2,000 over 30 years at 6.5% supports a loan of $316,421.64.

The gap is $101,254.92 of loan and about $126,000 of house — from the same income, the same rate and the same term. Neither line is wrong; they answer different questions. The first is roughly what an accommodating lender might approve. The second is what the 28/36 rule actually recommends. Notice too that the tool’s $2,640 of principal and interest plus $520 of escrow comes to $3,160 a month, which is 35.11% of gross income — well past the 28% housing guideline even though it passes the 36% one.

What a rate change does to your budget

Your income fixes the payment; the rate decides how much loan that payment buys. Holding a $2,000 principal-and-interest budget over 30 years:

Each extra percentage point costs roughly a tenth of your purchasing power, and the effect compounds across the range: the same payment buys $92,136.24 less loan at 8.5% than at 5.5%. This is why a pre-approval based on last quarter’s rates is not a budget. Re-price with the mortgage calculator when your rate lock is quoted, and remember it reports principal and interest only — escrow items are added by the servicer on top.

From a loan amount to a house price

Affordability tools return a loan, not a price. The price is the loan plus your down payment, which is what the down payment calculator is for: it splits a purchase price into the cash you put in and the loan you take out. Working on a $400,000 home at 6.5% over 30 years:

Dropping from 20% to 10% down frees $40,000 of cash and costs $432.82 a month — $252.82 of extra interest and principal, plus mortgage insurance that buys you nothing. Whether that trade is worth taking depends on what else the $40,000 is for. If it is your emergency fund, keep it and buy less house; if the alternative is waiting three more years, the surcharge may be the cheaper option, and PMI can usually be removed once you have built enough equity.

The costs no ratio captures

Debt-to-income ratios only see obligations a lender can verify on a credit report. Ownership adds a second set of costs that never appear in any ratio and are the usual reason a budget that looked comfortable on paper stops working in month eight.

• Closing costs are due on top of the down payment, in cash, at the worst possible moment for your bank balance.
• Maintenance and replacement. Roofs, water heaters and HVAC units fail on their own schedule. Budget a monthly reserve rather than discovering the number in an emergency.
• Everything that scales with square footage — utilities, furnishing, longer commutes from cheaper suburbs. A bigger house is more expensive than its mortgage.
• Tax and insurance drift. Escrow is re-estimated periodically, so a payment that fits today can rise even on a fixed-rate loan.

Setting your own number in five steps

1. Write down gross monthly household income, then multiply by 0.28 and by 0.36. Those two figures bracket every decision that follows.
2. Subtract your verifiable monthly debt payments from the 36% figure. If the result is smaller than the 28% figure, your other debts — not the housing market — are what is limiting you, and clearing the smallest of them buys back purchasing power immediately.
3. Get local property tax and homeowners insurance estimates for the price band you are shopping, add HOA dues where relevant, and subtract the total from your housing ceiling. What remains is your true principal-and-interest budget.
4. Convert that budget into a loan at a rate you can actually be quoted, then add your down payment to get a price. Re-run it a quarter-point higher as a stress test.
5. Check the leftovers. After the down payment and closing costs, do you still hold several months of expenses in cash? If not, the honest answer is a cheaper house or a later purchase, not a bigger loan.

In the worked example those five steps land on $316,421.64 of loan against the 36% test’s $417,676.56 — 24.2% lower. Expect a gap of that order, and expect it to feel like an unnecessary sacrifice right up until the first year of ownership, when it turns out to be the entire margin between owning a house and being owned by one.

Take a target, not a ceiling

Both halves of 28/36 are limits, and limits make poor plans. Three positions worth holding. First, if you carry no other debt, still cap housing near 28% rather than spending the whole 36% — the slack is what absorbs a job change or a new baby. Second, shop the price you can service on one income if you are buying on two. Third, decide the term on total interest rather than the payment: $320,000 at 6.5% costs $2,022.62 a month over 30 years and $2,787.54 over 15, but total interest falls from $408,142.36 to $181,757.84 — $226,384.52 saved for $764.92 more a month. If that payment fits inside 28%, the shorter term is the better buy.

Once you own the house, the same arithmetic runs in reverse when rates fall — the refinance break-even guide covers that — and if you are comparing lenders rather than prices, APR versus interest rate is the number that settles it.

Before you make an offer

Get a written escrow estimate for the specific property, not a regional average; confirm HOA dues and any special assessments; ask what PMI would cost and what removes it; and check that your payment still fits if the rate you lock is a quarter-point worse than quoted.

Every figure here is a planning estimate built from the rates and costs you type in. Approval depends on credit history, documented income, the appraisal and each lender’s own ratio limits, and your lender’s amortization schedule and closing disclosures govern the real numbers.