The 28/36 Rule, Explained Without the Fluff

The 28/36 rule is the oldest affordability heuristic in housing finance, and despite a hundred fintech apps trying to replace it, the underlying numbers still hold up. The rule has two parts, and most online affordability calculators only show you the first part — which is the one that usually overestimates what you can really afford.

The first ratio: 28 percent housing

The first ratio says your total monthly housing payment — principal, interest, property taxes, and homeowners insurance, often abbreviated PITI — should not exceed 28 percent of your gross monthly income. On a household earning $10,000 a month before taxes, that caps the housing payment at $2,800.

This is the number that mortgage calculators almost always lead with, because it produces the largest "affordable" home price and therefore the most engagement. But the 28 percent ratio assumes you have no other debt, which is almost never true.

The second ratio: 36 percent total debt

The second ratio is the one that actually keeps people out of trouble. Total monthly debt payments — housing plus car loans, student loans, credit card minimums, and any other recurring debt — should not exceed 36 percent of gross monthly income.

If that same $10,000 a month household has a $400 car payment and $300 in student loan payments, the total non-housing debt is $700. That leaves $2,900 (36 percent of $10,000 minus $700) for the housing payment — coincidentally close to the 28 percent number. But if the car payment is $600 and the student loan is $500, the housing budget drops to $2,500, and the 28 percent number is now misleadingly high.

Why lenders sometimes approve you for more

Modern mortgage underwriting will often approve borrowers up to 43 percent total debt-to-income, and government-backed programs like FHA can stretch even higher. This is not a sign that the 28/36 rule is outdated. It is a sign that lenders are pricing in slightly higher default risk and are willing to take it. You are the one paying the mortgage, not the lender, so the more conservative number is the more honest one.

What the rule does not include

• Maintenance. Plan on 1 to 2 percent of the home value per year. A $400,000 home is $4,000 to $8,000 a year, on top of the mortgage payment.
• HOA fees. If the home is in an HOA, fold the monthly dues into the housing payment when running the 28 percent test.
• Retirement contributions. The 36 percent ratio uses gross income, but most people should still be saving 10 to 15 percent of gross for retirement. A mortgage that crowds that out is unaffordable, regardless of what the ratios say.

Our 28/36 Affordability Check runs both ratios at once and tells you the binding constraint — usually it is the second one.