The 28/36 Rule Says You Can Borrow $406,000. The Real Number Is $300,000.

Ask a lender how much house you can afford and the answer will come from two ratios that have governed mortgage underwriting for decades. They are simple, they are useful, and they are routinely applied in a way that overstates what you can actually borrow by a very large margin.

What the 28/36 rule says

• The front-end ratio: your total monthly housing cost should not exceed 28% of gross monthly income.
• The back-end ratio: all monthly debt payments combined, including housing plus car loans, student loans, and credit card minimums, should not exceed 36% of gross monthly income.

Whichever limit binds first is the one that applies. Note that both use gross income, before tax and before retirement contributions, which is the first reason the resulting figure feels larger than what your bank account can support.

Working it through on $110,000

Gross monthly income is $9,166.67. The 28% front-end limit allows $2,566.67 for housing. The 36% back-end limit allows $3,300 for all debt combined.

Notice that $400 of existing debt changes nothing: the back-end limit still leaves $2,900 available for housing, which is more than the front-end limit permits. Debt only reduces your budget once it grows large enough that 36% minus your debts falls below 28%. On this income, that happens above roughly $733 of monthly obligations, about one car payment.

The omission that costs you $105,000

Here is where most online affordability tools go wrong. The 28% front-end limit covers your total housing cost, not just your mortgage payment. That means principal, interest, property taxes, homeowners insurance, and any HOA dues. The industry calls it PITI, and lenders apply it strictly.

Property tax and insurance are not small. On a home in the $350,000 to $400,000 range, $6,000 of annual property tax and $2,000 of annual insurance is entirely ordinary. That is $666.67 a month that must come out of the $2,566.67 before a single dollar reaches the mortgage.

The correct figure is $300,601. The figure a calculator produces by ignoring escrow is $406,074. The gap is $105,473, more than a quarter of the loan, and it is entirely an artifact of leaving out two line items that appear on every mortgage statement.

Why the ceiling is not the target

Even the corrected $300,601 is a maximum, not a recommendation. The ratios describe what a lender will approve, and lenders are assessing the risk that you default, not whether you will be able to save for retirement, replace a car, or absorb a period of reduced income.

Several substantial costs sit entirely outside the calculation:

• Maintenance and repairs. A widely used planning figure is 1% to 2% of the home's value annually. On a $375,000 house, that is $312 to $625 a month for something that never appears on a mortgage statement and cannot be deferred indefinitely.
• Private mortgage insurance. With less than 20% down on a conventional loan, PMI typically adds 0.3% to 1.5% of the loan balance per year, and lenders count it inside the 28%.
• Utilities. Moving from a two-bedroom apartment to a house frequently doubles them, and the buyer's budget rarely reflects it.
• Retirement contributions. The ratios use gross income, so a borrower contributing 10% to a 401(k) has meaningfully less available than the arithmetic implies.
• Income volatility. The ratios treat a commission-based income and a salaried income identically at the same annual figure.

This is why many financial planners suggest targeting closer to 25% of gross income for total housing, and why buyers who deliberately shop one price bracket below their approval tend to report far less stress about the decision two years later.

What lenders look at beyond the ratios

Passing 28/36 does not guarantee approval, and failing it does not guarantee rejection. Underwriting considers several other factors, and some allow meaningful flexibility:

1. Credit score. It drives your rate, and the rate drives how much loan a given payment supports. On a $300,000 loan, the spread between excellent and merely fair credit can easily exceed a full percentage point.
2. Down payment. More equity reduces the lender's risk and can support approval at higher ratios.
3. Cash reserves. Several months of payments in accessible savings after closing materially strengthens an application.
4. Loan program. FHA loans frequently permit back-end ratios above 43%, and qualified mortgages allow up to 43% as a general matter. Automated underwriting systems approve higher ratios routinely when compensating factors are present.
5. Employment stability. Two years in the same field is the conventional benchmark; recent self-employment complicates documentation considerably.

The practical approach: get pre-approved to learn what you can borrow, then decide independently what you want to borrow. Those are different questions, and only the second one is yours to answer.