The 28/36 Rule: The Mortgage Formula Every Buyer Should Know
Most people guess at their home budget. Lenders don't. Here's the affordability framework that mortgage underwriters have used for generations — and how to apply it to your income before you start house hunting.
Most people figure out their home budget by working backward from a monthly payment that feels manageable. That's not a strategy — that's a guess dressed up as a plan. Lenders don't work that way, and neither should you.
The 28/36 rule is the affordability framework that mortgage underwriters have leaned on for generations. It's not complicated, it's not outdated, and understanding it before you start house hunting could save you from buying a home that quietly breaks your budget six months after you close.
What Is the 28/36 Rule?
The 28/36 rule sets two spending ceilings based on your gross monthly income — the income you earn before taxes, retirement contributions, or any other deductions come out.
The 28% front-end ratio: Your total monthly housing costs — mortgage principal, interest, property taxes, and homeowner's insurance (PITI) — should not exceed 28% of your gross monthly income.
The 36% back-end ratio: Your total monthly debt obligations — housing costs plus all other recurring debts like car loans, student loans, and credit cards — should not exceed 36% of your gross monthly income.
Both numbers have to work simultaneously. A lot of buyers focus only on the mortgage payment and forget the back-end ratio entirely — right up until a lender flags their debt load and the pre-approval comes in lower than expected.
How to Calculate the 28/36 Rule for Your Income
Start with your gross monthly income — annual salary divided by 12.
36% back-end limit: $2,700/month (maximum total debt)
Now subtract existing monthly debts from that $2,700 ceiling:
Debt
Monthly Payment
Car loan
$450
Student loans
$300
Credit card minimums
$75
Total existing debt
$825
Remaining room for a mortgage: $2,700 − $825 = $1,875/month
Notice that's already below the front-end limit of $2,100. The back-end ratio is the binding constraint — which is exactly why carrying existing debt before buying a home costs you more purchase power than most buyers realise.
Enter your income, debts, and down payment below. The calculator applies both the 28% and 36% thresholds automatically and gives you a real purchase price ceiling.
28/36 Rule — Home Affordability Calculator
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Front-End Ratio vs. Back-End Ratio: Which One Matters More?
Both matter — but in different ways and at different stages of the buying process.
Front-End Ratio: Your Day-to-Day Breathing Room
The front-end ratio (28%) governs your monthly cash flow. Stay within it and your housing costs remain a predictable, manageable slice of your income. Push past it and you may qualify on paper but feel house-poor in practice — technically affording your mortgage while cutting back on everything else.
Back-End Ratio: What Lenders Actually Scrutinise
The back-end ratio (36%) is what underwriters focus on most heavily during loan approval. Most conventional loans require a debt-to-income (DTI) ratio of 43% or below, though 36% is the conservative benchmark. FHA loans can stretch to 50% in some cases, but higher DTI typically means stricter conditions and less favourable terms.
⚠️ Qualifying vs. Comfortably Affording
A lender approving you at 43% DTI isn't telling you that's a healthy financial position — they're telling you it clears their minimum threshold. Qualifying for a loan and comfortably affording a loan are two very different things.
When the 28/36 Rule Doesn't Perfectly Apply
The rule was developed in an era of more uniform housing costs. Today, strict adherence isn't always realistic — or even necessary.
High-Income Earners
A household earning $250,000 a year has significant discretionary income even after housing. Spending 33% of gross income on housing still leaves enormous room for savings and lifestyle. For higher earners, the absolute dollar amounts matter more than the percentages.
High-Cost Housing Markets
In San Francisco, Seattle, Boston, or New York, median home prices relative to median incomes make 28% an almost impossible target for average earners. Many financially stable buyers in these markets operate closer to 35–40% front-end ratios and manage fine — because they have minimal other debt, stable high-demand careers, and strong savings.
Low Debt Loads
A buyer with zero car payments, no student loans, and no credit card balances has a fundamentally different risk profile than someone with $1,200/month in existing obligations at the same income. If your back-end ratio is well under 36% even with a slightly higher housing payment, the 28% ceiling has more flexibility. The rule is a starting framework, not a ceiling carved in stone.
Real-World Scenario: Same Income, Different Outcomes
Two buyers, both earning $85,000 a year. Same income — very different mortgage capacity.
✓ Buyer A — Low debt
$85,000 salary
Car payment$0
Student loans$0
Credit cards$200/mo
Total existing debt$200/mo
Mortgage capacity: ~$2,660/month
⚠ Buyer B — Existing debt
$85,000 salary
Car payment$550/mo
Student loans$400/mo
Credit cards$150/mo
Total existing debt$1,100/mo
Mortgage capacity: ~$1,960/month
That $700/month difference in mortgage capacity translates to roughly $60,000–$80,000 less in purchase price. Both buyers earn identical salaries. Buyer B isn't doing anything wrong — but their existing debt has quietly reshaped what homeownership looks like for them. Knowing this before touring homes at the wrong price point is the entire value of running this math early.
How to Use the 28/36 Rule Before You Start Shopping
Don't wait for a lender to run these numbers. Do it yourself first:
Calculate your gross monthly income — salary divided by 12
Multiply by 0.28 — this is your housing payment ceiling
Multiply by 0.36 — this is your total debt ceiling
Add up all existing monthly minimum debt payments
Subtract existing debts from the 36% ceiling — the result is your realistic mortgage limit
Use whichever number is lower — the 28% result or the adjusted 36% result
Then run that monthly payment through a mortgage calculator to find the corresponding purchase price at current interest rates. That number — not a bank's pre-approval letter — is your honest starting point.
Is the 28/36 rule the same as the debt-to-income ratio?
They overlap but aren't identical. DTI typically refers to the back-end ratio only — total debt as a percentage of gross income. The 28/36 rule includes both the front-end (housing only) and back-end (total debt) thresholds together as a paired framework.
What if I exceed the 28/36 rule — will I be denied a mortgage?
Not necessarily. Conventional lenders commonly approve loans up to 43% back-end DTI, and FHA loans allow even higher. Exceeding 28/36 means more risk and less monthly breathing room — not an automatic denial.
Does the 28/36 rule apply to gross or net income?
Gross income — before taxes and deductions. Lenders universally use pre-tax income for these calculations. When assessing what you can comfortably live with day-to-day, it's worth running the same numbers against your take-home pay as a secondary check.
Should I include HOA fees in my 28% calculation?
Yes. HOA fees are a fixed monthly housing cost and many lenders include them in the front-end ratio calculation. Always factor them in when evaluating a condo or community with association fees.
How does the 28/36 rule work with variable income?
Lenders typically average variable income — commissions, freelance, bonuses — over 24 months. Use that averaged figure as your gross monthly income when running the 28/36 calculation for a realistic pre-approval picture.
Run Your Numbers Before You Run Your Search
The 28/36 rule gives you a clear, honest starting point — before you're emotionally attached to a home or a price point. Use the calculator above to apply it to your exact situation.