Home Affordability: The 28/36 Debt-to-Income (DTI) Rule, PITI & Maximum Purchase Power
Knowing how much house you can afford is not simply a matter of multiplying your salary by four. Mortgage lenders evaluate risk through two critical metrics: the Front-End DTI Ratio (housing expense) and the Back-End DTI Ratio (total debt burden). Understanding the traditional 28/36 rule and modern conforming loan caps (up to 45% or 50% DTI) protects you from becoming "house poor."
format_list_bulleted Table of Contents
- 1. Front-End vs Back-End Debt-to-Income (DTI) Explained
- 2. The 28/36 Underwriting Rule (Formulas & Worked Example)
- 3. Deconstructing PITI: Principal, Interest, Taxes & Insurance
- 4. Maximum DTI Thresholds by Loan Type (Conventional, FHA, VA, Jumbo)
- 5. The Post-Closing Liquidity Buffer: Emergency Reserves
1. Front-End vs Back-End Debt-to-Income (DTI) Explained
Lenders calculate DTI as a percentage of your Gross Monthly Income (pre-tax):
- Front-End DTI (Housing Ratio): Only includes complete housing expenses: Monthly Principal, Interest, Property Taxes, Homeowners Insurance, HOA dues, and PMI (PITI).
- Back-End DTI (Total Obligation Ratio): Includes PITI plus all recurring minimum debt payments (auto loans, student loans, minimum credit card payments, personal loans, child support).
2. The 28/36 Underwriting Rule (Formulas & Worked Example)
The gold standard benchmark states that you should spend no more than 28% of gross monthly income on housing, and no more than 36% on total recurring debts:
• Max Housing Expense (28% Front-End): $10,000 × 0.28 = $2,800 / month (Max PITI)
• Max Total Debt (36% Back-End): $10,000 × 0.36 = $3,600 / month
If the household has $600 in car/student loan payments, their available housing budget is $3,600 − $600 = $3,000/mo (capped at $2,800 by the front-end rule).
3. Deconstructing PITI: Principal, Interest, Taxes & Insurance
When an online calculator estimates a $2,100 monthly mortgage, that is only Principal & Interest. Your actual lender escrow payment includes:
- Principal & Interest: $2,100
- Property Taxes (1.25% avg): +$450 / month
- Homeowners Hazard Insurance: +$150 / month
- HOA Dues (if applicable): +$100 / month
- Total True Monthly Outflow (PITI): $2,800 / month
4. Maximum DTI Thresholds by Loan Type
While 28/36 is recommended for long-term safety, automated underwriting systems allow higher limits:
| Loan Program | Standard DTI Cap | Max with Compensating Factors |
|---|---|---|
| Conventional (Fannie/Freddie) | 36% | 45% – 50% |
| FHA Loans | 31% / 43% | 46.9% / 56.9% |
| VA Loans (Veterans) | 41% | No hard cap (Residual Income test) |
| Jumbo Loans | 38% – 43% | 43% strict limit + 12 mo reserves |
5. The Post-Closing Liquidity Buffer
Never drain 100% of your savings for a down payment. Closing costs add 2% to 5% ($8,000 to $20,000), and home emergencies inevitably strike within the first 12 months. Ensure you retain at least 3 to 6 months of living expenses in a high-yield cash reserve.