The 28/36 Rule for Mortgages: How Lenders Calculate What You Can Afford

The 28/36 rule is the foundational affordability guideline in US mortgage underwriting. Understanding it — including where it's rigid and where lenders bend it — tells you exactly how your maximum loan amount gets calculated, and why the bank's number often isn't the right number for your life.

What the 28/36 Rule Means

The 28/36 rule establishes two simultaneous limits on how much debt a household should carry:

28% front-end (housing expense ratio): Your total monthly housing cost — PITI: principal, interest, property taxes, and homeowners insurance — should not exceed 28% of your gross monthly income.

36% back-end (total debt-to-income ratio): All monthly debt obligations combined — housing payment plus minimum payments on all other debts (credit cards, auto loans, student loans, personal loans) — should not exceed 36% of your gross monthly income.

Both limits apply simultaneously. Whichever is more restrictive determines your actual ceiling.

The Math in Practice

Example: Household gross income of $9,500/month.

Front-end calculation: Maximum PITI = $9,500 × 0.28 = $2,660/month

Back-end calculation: Total maximum debt = $9,500 × 0.36 = $3,420/month Less existing monthly debt (car loan $450, student loan $280) = $730 Maximum housing payment = $3,420 − $730 = $2,690/month

The binding constraint: front-end at $2,660/month (slightly more restrictive). After estimated taxes ($480) and insurance ($150), available P&I budget is approximately $2,030.

At 7.0% for 30 years, $2,030/month supports a loan of approximately $305,000. Add 10% down ($33,900) → maximum purchase price around $338,900.

Notice that the $730/month in existing debt reduces the back-end calculation but the front-end limit is hit first anyway. If existing debt were higher — say $1,200/month — the back-end limit would bind instead: $3,420 − $1,200 = $2,220 available for housing.

How the Rule Interacts with Real Mortgage Underwriting

The 28/36 rule is a guideline, not a hard regulatory cap. In practice:

Automated underwriting extends the limits: Fannie Mae's Desktop Underwriter (DU) and Freddie Mac's Loan Prospector (LP) approve borrowers based on holistic risk profiles, not just DTI. Borrowers with high credit scores (760+) and significant reserves regularly get approved to 43–45% back-end DTI, well above 36%.

FHA loans go to 57%: The Federal Housing Administration allows back-end DTIs up to 57% with strong compensating factors. The front-end limit for FHA is 31%.

Conventional limits are 36–45% in practice: The strict 36% back-end figure is the conservative guideline. Real approval thresholds in the US conventional market run to 43%, and sometimes 50% for highly qualified borrowers.

The 28% front-end is rarely the binding constraint: In high-cost markets, property taxes and insurance consume a significant portion of the housing budget. A $2,660 front-end limit in New Jersey, where property taxes on a typical home run $8,000–$12,000/year ($667–$1,000/month), leaves only $1,660–$1,993 for P&I — substantially reducing the viable loan amount.

Why the 36% Number Exists

The 36% limit wasn't chosen arbitrarily. It emerged from decades of default data showing that households spending more than 36% of gross income on total debt service faced meaningfully higher rates of financial distress and default.

But gross income is a blunt measure. Consider two households both earning $10,000/month gross:

Household A: Lives in Texas (no state income tax), invests nothing in a retirement plan. Take-home: approximately $7,800/month.

Household B: Lives in California (13.3% top bracket), maxes a 401(k) ($2,292/month in contributions). Take-home: approximately $5,400/month.

At 36% back-end DTI, Household A's monthly debt service of $3,600 represents 46% of take-home pay. For Household B, $3,600 represents 67% of take-home — essentially insolvent.

The 28/36 rule treats both households identically. It's a lender risk model, not a personal affordability model.

A More Realistic Affordability Target

Financial planners often recommend a more conservative personal guideline:

• Housing costs (PITI + utilities + maintenance reserve): 25–30% of gross income
• Total debt service: 30–36% of gross income

The maintenance reserve matters. A $400,000 home requires $4,000–$8,000/year (1–2% of value) in ongoing maintenance. That's $333–$667/month that doesn't appear anywhere in the PITI calculation but is a real, recurring cash outflow.

A practical personal affordability calculation:

1. Start with gross monthly income
2. Estimate take-home after federal/state taxes and retirement contributions
3. Calculate your comfortable total housing spend as a percentage of take-home (not gross) — aim for under 35% of take-home
4. That becomes your real budget ceiling, which then converts to a purchase price

For most middle-income buyers, this personal budget ceiling comes in below the lender's maximum approval.

The 28/36 Rule in Other Markets

United Kingdom: UK lenders use income multiples (4–4.5x annual income) rather than percentage ratios. The implicit housing-cost-to-income ratio varies by income level. Higher earners borrowing 4x income may have lower percentages of income going to housing; lower earners borrowing 4x have a higher implicit ratio.

Canada: Gross Debt Service (GDS) ratio should not exceed 39% of gross income. Total Debt Service (TDS) ratio should not exceed 44%. These are functionally similar to the US 28/36 rule but with higher thresholds, partially offset by the mandatory stress test that reduces the loan qualifying amount.

Australia: No standardized DTI threshold, but APRA caps aggregate lending where borrower DTI exceeds 6x income (including the loan being applied for) to 20% of total new loan volume. Serviceability ratios of 30–35% of gross income for housing costs are typical guidelines.

New Zealand: The RBNZ uses debt-to-income (DTI) limits — as of 2024–2025, no more than 20% of new owner-occupier loans can have DTI above 6, and no more than 20% of investor loans can have DTI above 7.

Using the Rule as a Filter, Not a Ceiling

The 28/36 rule works well as an initial filter: if the purchase you're considering would push you above both thresholds simultaneously, the lender will likely decline the loan.

It works poorly as a target: running straight to the limit maximizes your loan amount but minimizes your financial breathing room.

The practical approach is to run both calculations — what the lender will approve, and what genuinely fits your take-home income and lifestyle expenses — then make an offer somewhere between the two based on your priorities, income stability, and risk tolerance.

The Mortgage Math & Affordability Calculator Toolkit includes a full DTI worksheet that calculates both the front-end and back-end ratios for any income/debt/payment combination, identifies the binding constraint, and models multiple purchase price scenarios — including a personal affordability calculation based on take-home pay rather than gross income, which is where most buyers actually feel the budget.

The Number That Matters Most

The most important number in your home purchase decision is not what the bank approves. It's the monthly payment you can sustain for 30 years across a range of life events: job change, medical expenses, kids, market downturns, a roof replacement.

The 28/36 rule tells you the maximum; prudence tells you to aim at something meaningfully below it.