The 28/36 Rule Explained: How Much House Can You Really Afford?

Walk into any mortgage broker's office in the United States and you will hear some version of the 28/36 rule within the first ten minutes. It is one of the oldest, simplest, and most durable yardsticks in American household finance: keep your housing payment under 28% of gross monthly income, and your total debt payments under 36%. Here is where it came from, why lenders still use it, and how to adjust it for the way Americans actually live in 2026.

Where the rule came from

The 28/36 thresholds trace back to the underwriting standards Fannie Mae and Freddie Mac codified in the 1970s and 1980s. They were designed to identify borrowers whose income left enough room for property taxes, insurance, food, transportation, and savings after the mortgage was paid. The rule has been challenged repeatedly, but lenders keep returning to it because it does a remarkably good job of predicting default risk across decades and across rate environments.

How to compute your numbers

• Front-end ratio (housing): principal + interest + property tax + homeowners insurance + HOA, divided by gross monthly income. Target: below 28%.
• Back-end ratio (total debt): everything in the front-end ratio PLUS minimum payments on credit cards, student loans, auto loans, and any court-ordered support, divided by gross monthly income. Target: below 36%.
• Gross monthly income is before tax. If your household earns $120,000 a year, your monthly figure is $10,000.

A worked example

A couple earning $120,000 gross has $10,000 a month to work with. The 28% front-end cap puts their maximum housing payment at $2,800. The 36% back-end cap puts their maximum total debt at $3,600. If they already have $700 of student loans and a $400 car payment, they have $2,500 of headroom for housing — meaningfully less than the 28% would suggest. The back-end ratio almost always binds first for households with student debt.

Why lenders apply stricter overlays

Conventional conforming loans technically allow back-end ratios up to 45% (and FHA up to 50%), but those higher ratios usually require compensating factors: a credit score above 740, a 20% down payment, or 12 months of cash reserves. The 28/36 rule is what underwriters use when they want an automatic approval with no extra documentation.

Modern adjustments worth making

• Use take-home pay, not gross, for the front-end check. American tax brackets, FICA, state tax, and health insurance leave many households with only 70–75 cents on the gross dollar.
• Include child care. For a household with two kids in full-time care, child care can rival the mortgage payment and is not in the standard 28/36 formula.
• Stress-test for a rate reset. If you are buying with a 7/1 ARM, run the affordability check at the fully-indexed rate, not the teaser.
• Subtract retirement savings. A household contributing 15% of gross to a 401(k) is effectively living on 85% of income; the 28% cap should apply to that smaller base.

Our Mortgage & Affordability Calculator implements the classic 28/36 rule and surfaces your maximum home price in seconds. Use it as a starting point, then apply the modern adjustments above to land on the number that actually fits your life.

Sources: Fannie Mae Single Family Selling Guide, Consumer Financial Protection Bureau ability-to-repay rule, and Freddie Mac underwriting guidelines.