The 28/36 Rule: How to Calculate the Home Price You Can Actually Afford

The formula lenders use to decide what house you can afford — and how to run it yourself.

Most first-time buyers walk into a lender’s office with a number in their head — what they think they can afford based on their monthly take-home pay and a quick mental check of the rent they’ve been paying. That number is usually wrong.

The real number comes from a formula lenders have been using for decades: the 28/36 rule. It’s a debt-to-income (DTI) test that decides whether you’ll get approved, how much you’ll be approved for, and — if you ignore it — whether you’ll end up house-poor a year into ownership.

This post walks through exactly how the 28/36 rule works, how to calculate both sides of it for your own income, and how to figure out the maximum home price the math actually supports. We’ll work through real numbers, not vague guidance.

What the 28/36 rule is

The 28/36 rule is two ceilings, applied at the same time.

• Front-end DTI: 28% — Your total monthly housing payment should be no more than 28% of your gross (pre-tax) monthly income.
• Back-end DTI: 36% — Your housing payment plus every other monthly debt obligation should be no more than 36% of gross monthly income.

The key word in both is gross. Lenders don’t care about your take-home pay after taxes, 401(k) contributions, and health insurance. They look at what’s on your W-2 before any of that comes out.

Whichever ratio caps you first sets your budget. For someone with no debt, the 28% front-end limit usually binds. For someone with a car loan and student loans, the 36% back-end limit often binds well before they hit the 28% on housing alone.

What counts as “housing payment”

The housing payment in the 28/36 rule isn’t just principal and interest. Lenders use PITI:

• Principal — the amount paying down your loan balance
• Interest — the cost of borrowing
• Taxes — property taxes, divided into monthly chunks
• Insurance — homeowners insurance, also monthly

If you’re buying a condo, townhome, or anything with a homeowners association, HOA dues get added to PITI too. Some people call this PITIA. Either way, lenders include them in the housing payment.

This matters because people frequently look at the principal-and-interest payment on a mortgage calculator and think they’ve found their number. They haven’t. Taxes and insurance in many parts of the country add 25–40% on top of P&I. In New Jersey, Illinois, and Texas — where property tax rates run above 2% — they can add even more.

What counts as “other debt”

The other debt side of the 36% back-end ratio includes the minimum required payments on:

• Auto loans
• Student loans
• Credit card minimums
• Personal loans
• Any other installment debt reporting to your credit
• Court-ordered payments (child support, alimony)

It does not include groceries, utilities, gas, subscriptions, daycare, or any other recurring expense that doesn’t show up on your credit report. Lenders aren’t measuring your budget — they’re measuring what creditors have a claim on.

This distinction matters. You can have $2,000 a month in daycare costs and the lender’s math won’t reflect it. That’s where the gap between “qualifying for a mortgage” and “affording a mortgage” opens up, which we’ll come back to.

Calculating front-end DTI

Front-end DTI is the simpler of the two. The formula:

Front-end DTI = Monthly housing payment (PITI) ÷ Gross monthly income

Run it backward to find what housing payment fits inside 28%:

Max monthly housing payment = Gross monthly income × 0.28

Example: a household earning $90,000 a year has gross monthly income of $7,500. The 28% ceiling is $7,500 × 0.28 = $2,100. That’s the maximum monthly PITI the front-end rule allows.

Note what this number isn’t telling you yet: the home price. It’s telling you the monthly payment ceiling. To convert that into a home price, you have to back out the taxes, insurance, and HOA — then run the remaining principal-and-interest amount through a mortgage formula to find the loan size, then add your down payment. That’s where a calculator earns its keep.

Calculating back-end DTI

Back-end DTI is the same formula with all other debt added in:

Back-end DTI = (Housing payment + all other monthly debts) ÷ Gross monthly income

Worked backward to find what housing payment fits inside 36%:

Max housing payment (back-end) = (Gross monthly income × 0.36) − Existing monthly debt

Continuing the example: that same $7,500 gross income gets a 36% ceiling of $2,700. If the buyer has a $400 car payment and $200 in student loan minimums — $600 in existing debt — the housing payment can be at most $2,700 − $600 = $2,100.

In this case, both rules land on $2,100. The buyer is equally constrained by front-end and back-end DTI.

Now imagine the same buyer with $1,200 in existing monthly debt instead. The back-end ceiling drops to $2,700 − $1,200 = $1,500. Now the back-end rule binds — they can only afford a $1,500 housing payment, not $2,100. Existing debt has cost them $600 a month of borrowing power, which translates to roughly $90,000 to $100,000 of home price at current rates.

This is why mortgage advisors so often tell buyers to pay off a car loan before applying. It typically frees up more buying power than putting another $20,000 toward a down payment would. If you’re carrying meaningful debt going into a home purchase, our debt payoff calculator shows how aggressively you’d need to attack it to free up borrowing power before applying.

Putting it together: a worked example

Let’s run a complete calculation for a household with the following profile:

• Gross income: $120,000/year ($10,000/month)
• Existing debt: $500/month (car) + $250/month (student loans) = $750
• Down payment: $40,000
• Mortgage rate: 7.0%
• 30-year fixed loan
• Property tax: 1.2% of home value annually
• Homeowners insurance: 0.35% of home value annually
• No HOA

Step 1: Apply the 28/36 ceilings.

• Front-end max housing payment: $10,000 × 0.28 = $2,800
• Back-end max housing payment: ($10,000 × 0.36) − $750 = $2,850

The front-end rule binds. The housing payment ceiling is $2,800.

Step 2: Back out taxes and insurance to find principal-and-interest budget.

Property tax and insurance combined are 1.55% of home value annually, or roughly 0.129% monthly. For a home priced at H, the monthly tax-and-insurance cost is H × 0.00129.

So: P&I + (H × 0.00129) = $2,800

Step 3: Solve for home price.

This is the part a calculator does in milliseconds and the manual version takes a while. The mortgage payment formula relates P&I to loan amount (home price minus down payment), interest rate, and term:

P&I = L × [r(1+r)^n] / [(1+r)^n − 1]

Where L is loan amount, r is the monthly interest rate (annual / 12), and n is the number of monthly payments (30 × 12 = 360).

Plugging $40,000 down payment, 7% rate, and 30-year term into the combined equation and solving for H gives roughly $430,000 as the maximum affordable home price.

This is the number our mortgage affordability calculator returns instantly when you enter the same inputs.

When the 28/36 rule doesn’t quite fit

The 28/36 rule is the conventional standard. It’s not the only one lenders use.

• FHA loans allow front-end ratios up to 31% and back-end ratios up to 43% (sometimes higher with compensating factors). The tradeoff is mandatory mortgage insurance premiums for the life of most FHA loans.
• VA loans don’t enforce a strict front-end ratio. They focus on back-end DTI and residual income — how much cash is left after all monthly obligations.
• Jumbo loans for higher-priced homes often impose stricter limits than 28/36, sometimes requiring back-end DTI under 35% along with significant cash reserves.
• Conventional loans with strong compensating factors — high credit, large down payment, substantial reserves — can sometimes push back-end DTI to 45% or 50%.

Even within conventional lending, the 28/36 rule is more of a default than a hard cap. But it’s the right starting point for planning, and it’s what most lenders use to give you a rough pre-qualification number.

Qualifying isn’t the same as affording

This is the part most articles skip.

The 28/36 rule will tell you what a lender will approve. It won’t tell you what you can comfortably live with. A back-end DTI of 36% looks fine on paper, but if you also spend 6% of your income on childcare, 5% on health insurance not deducted from your check, and 8% on transportation costs, you’re suddenly running tight before you’ve bought a single grocery.

A useful gut check: take your front-end DTI ceiling and ask yourself whether you’d be comfortable spending that much on housing for the next ten years. If a $2,800 monthly housing payment feels like it would crowd out everything else — travel, retirement contributions, kid expenses, repairs — that’s a signal to buy below your maximum, not at it.

The lender’s math protects the lender. It tells them you’ll keep paying. It doesn’t tell you whether you’ll enjoy your life.

A reasonable rule of thumb among financial planners is to stay under 25% of gross income on housing if possible, especially with kids, irregular income, or any plans that need cash flow flexibility. Going to the full 28% — or the FHA’s 31% — is a choice, not a recommendation.

How to actually use this when shopping for a home

Three things are worth doing before you start looking at listings.

1. Calculate your maximum based on the 28/36 rule. Run your numbers through a mortgage affordability calculator using realistic interest rates and property tax assumptions for your area. This gives you the lender’s number.

2. Decide what fraction of that number you actually want to spend. If the math says $430,000 is your ceiling but you’d rather have room in the budget for other things, $350,000 might be a better target. The cushion is yours to keep.

3. Get pre-approved. A pre-approval letter is what real estate agents and sellers actually take seriously. The lender will run their own version of the 28/36 calculation, check your credit, verify employment, and issue a letter stating what they’ll lend. The number on that letter is your real ceiling — and it should ideally be close to what your own calculation produced.

If you’re not sure whether buying makes financial sense compared to renting given how long you plan to stay, the rent vs buy calculator compares the total cost of both paths over your time horizon. And once you know your target price, the loan payment calculator gives you the exact monthly principal and interest figure.

The 28/36 rule isn’t magic and it isn’t a guarantee. It’s a formula that has held up across decades of housing cycles because it leaves enough room in most household budgets to keep paying the mortgage when something goes wrong. Knowing your number — and how to calculate it — is the difference between guessing at a price range and shopping with real numbers behind you.