If you've read anything about mortgage affordability you've met the 28/36 rule: keep housing costs at or under 28% of your gross monthly income, and all your debt payments at or under 36%. It's the oldest and most widely referenced affordability guideline in American lending.
It's also routinely misapplied — people compare it against the wrong income, forget half of what belongs in the numerator, or treat it as a verdict rather than a starting point. Here's how it actually works.
The two ratios
Lenders call these your debt-to-income ratios, and there are two of them.
The front-end ratio (the 28) is your total monthly housing cost divided by gross monthly income. The back-end ratio (the 36) is all your monthly debt obligations — housing included — divided by the same income.
Both are calculated on the same denominator. The back-end ratio is always the larger number, because it contains everything the front-end ratio contains plus your other debts.
What counts as "housing cost"
This is where most self-assessments go wrong. The 28% is not your principal and interest payment. It's the full monthly cost of owning, usually abbreviated PITI:
- Principal — the part that reduces your loan balance.
- Interest — the lender's charge on the outstanding balance.
- Taxes — one twelfth of your annual property tax bill.
- Insurance — one twelfth of your annual homeowner's premium.
Plus, where they apply: mortgage insurance (PMI on a conventional loan under 20% equity), and any HOA, condo, or co-op fee. Those last two are genuinely part of the calculation and are the most commonly forgotten.
Someone who calculates 28% of their income, compares it against a principal-and-interest quote, and concludes they're comfortable can be well over the line once taxes, insurance and an HOA fee are added. In a high-property-tax state the difference is substantial.
What counts in the back-end 36%
Everything in the housing figure, plus the minimum required monthly payments on your other debts: car loans and leases, student loans, credit card minimums, personal loans, and court-ordered obligations like child support or alimony.
Two things that generally don't count: expenses that aren't debt — groceries, utilities, phone bills, childcare, insurance premiums other than the home's — and debts you'll have finished paying within a short window, which some lenders will exclude.
Note what this implies. Your grocery bill and childcare costs don't enter the lender's arithmetic at all, which is exactly why the rule can approve a payment you can't comfortably live with. More on that below.
Why gross income, not take-home?
The ratios use gross — pre-tax — income, which strikes most people as unrealistic. It is, a bit. The reason is standardisation: net pay depends on your tax filing status, state income tax, retirement contributions, and benefit elections, none of which are comparable between two applicants. Gross income is a consistent measure across every borrower in the country.
The practical consequence is that the rule is more generous to people in low-tax situations and less realistic for people in high-tax ones, and it says nothing about how much of your gross pay you divert to a 401(k). Two buyers with identical salaries in different states genuinely have different amounts of money available, and the 28/36 rule cannot see that difference.
Working the rule backwards
The rule is more useful run in reverse: start from income, derive a payment, derive a price.
Take gross annual income, divide by twelve for gross monthly income, and multiply by 0.28. That's your maximum monthly housing cost under the front-end rule. Now subtract your estimated monthly property tax, home insurance, and any HOA fee — what's left is what's available for principal and interest.
From that principal-and-interest figure, a mortgage calculator can solve for the loan amount at your rate and term, and adding your down payment gives a purchase price. That last step is exactly what the affordability tables on our state calculator pages do, using each state's real average property-tax rate.
Do the same arithmetic with 0.36 and subtract your other debt payments, then take whichever of the two answers is smaller. That's the constraint that actually binds.
Where the rule is stricter or looser than reality
The 28/36 thresholds are a guideline, not a regulation, and real underwriting frequently allows higher back-end ratios than 36% — particularly with strong compensating factors like a large down payment, substantial cash reserves, or an excellent credit history. Different loan programmes apply different tolerances.
So being over 36% doesn't automatically mean rejection. But it does mean you're relying on a lender's discretion rather than fitting comfortably inside the template, and you should expect more scrutiny.
The rule's real blind spot
Here is the thing worth internalising: the 28/36 rule measures what a lender is willing to risk, not what will leave you comfortable. Those are different questions, and only the second one is yours.
The ratios ignore childcare, healthcare, commuting, how much you save, whether your income is stable or variable, and how much maintenance the specific house will demand. A first-time buyer approved at the ceiling of the range can be technically qualified and practically stretched at the same time.
A more honest personal test is to run the full monthly housing cost against your actual take-home pay, subtract everything you genuinely spend, and see what's left over. If the answer is "nothing", the loan is affordable to the lender and not to you.
The bottom line
28% of gross income for full housing costs including taxes, insurance and HOA; 36% for all debt payments combined. Use gross income because that's what lenders use, work the rule backwards to get to a price, and take the lower of the two answers.
Then check it a second time against your real take-home pay and your real spending — because the rule was designed to protect the lender, and only you can run the version that protects you.