What the 28/36 DTI Rule Actually Measures
Lenders use a pair of ratios — the front-end and back-end debt-to-income (DTI) ratios — to determine how large a mortgage payment a borrower can safely carry. The conventional benchmark is 28 for the front-end and 36 for the back-end, though the exact thresholds vary by loan type, lender, and borrower profile.
Front-end ratio (housing ratio): The share of gross monthly income consumed by housing costs — principal, interest, property taxes, homeowner’s insurance, and, where applicable, PMI (private mortgage insurance) and HOA dues. Abbreviated as PITI (principal, interest, taxes, insurance).
Front-end DTI = Monthly PITI ÷ Gross Monthly Income × 100Back-end ratio (total DTI): The share of gross monthly income consumed by all recurring debt obligations — housing costs plus auto loans, student loans, credit card minimum payments, and any other installment or revolving debt.
Back-end DTI = (Monthly PITI + All Other Monthly Debts) ÷ Gross Monthly Income × 100The 28/36 standard means lenders typically want the housing payment to be no more than 28% of gross monthly income, and total debt payments (including housing) to be no more than 36%.
Both ratios are evaluated simultaneously. The binding constraint — the one that produces the lower maximum payment — governs how much home a borrower can qualify for.
How the Calculation Works Step by Step
Determining the maximum affordable home price from income and debt information involves a few interdependent steps:
Step 1: Compute the DTI payment limits.
Max housing payment (front-end): Income × 28% = $8,000 × 0.28 = $2,240
Max housing payment (back-end): Income × 36% − Other Debts = $8,000 × 0.36 − $500 = $2,388The binding limit is the smaller of the two: $2,240 (front-end governs here).
Step 2: Separate taxes and insurance from the loan payment.
The $2,240 covers PITI — total housing costs including taxes and insurance, not just the loan payment. A common rule of thumb for estimating property taxes and homeowner’s insurance together is approximately 1.25% of home value per year (0.104% per month). This fraction depends on the home’s value, which is what we’re solving for — so the calculation requires an iterative loop.
Step 3: Compute the maximum loan amount.
Once taxes and insurance are separated out, the remaining payment capacity services the loan itself (principal and interest). The standard mortgage payment formula gives the loan amount as the present value of that monthly payment stream:
Loan Amount = Payment × [(1 − (1 + r)^−n) / r]where r = monthly rate (annual rate ÷ 12) and n = number of payments (years × 12).
Step 4: Add the down payment.
Max Home Price = Max Loan Amount + Down PaymentWorked Example
A household with:
- $8,000 gross monthly income
- $500 in existing monthly debts (auto loan + minimum credit card payment)
- $60,000 down payment
- 7% annual mortgage rate, 30-year term
- Standard 28/36 DTI ratios
Step 1: DTI limits
| Limit | Calculation | Result |
|---|---|---|
| Front-end max PITI | $8,000 × 28% | $2,240 |
| Back-end max PITI | $8,000 × 36% − $500 | $2,388 |
| Binding limit | min($2,240, $2,388) | $2,240 |
The front-end ratio is the binding constraint.
Step 2–4: Solve for home price (iterative)
After iterating to account for the taxes-and-insurance fraction:
| Output | Value |
|---|---|
| Maximum home price | $342,987 |
| Maximum loan amount | $282,987 |
| Estimated monthly payment (PITI) | $2,240 |
| Monthly P&I only | $1,883 |
| Estimated taxes + insurance | $357 |
The household can afford a home priced up to approximately $343,000, with a $60,000 down payment and a 30-year loan at 7%.
How to Use the Home Affordability Calculator
The calculator implements the same 28/36 DTI logic, with a few inputs to fill in:
Gross monthly income: Pre-tax income from all sources — salary, self-employment, rental income, and any other income a lender would count. Use gross income (before taxes), not take-home pay. Lenders evaluate DTI on gross income because tax liability varies by household.
Monthly debts: The minimum required payment on every recurring debt obligation that will appear on a credit report: auto loans, student loan payments, personal loan installments, and the minimum payment on each credit card. Do not include utilities, groceries, or expenses that don’t appear as debt on a credit report.
Down payment: The amount available as a down payment, not including closing costs. A larger down payment reduces the loan amount and increases the maximum affordable home price for a given income level.
Annual rate and term: The expected mortgage interest rate and loan term. Rates vary by credit score, lender, loan type, and market conditions. A lower rate meaningfully increases purchasing power; a higher rate reduces it. The 30-year fixed is the most common US mortgage product for purchase financing.
Front and back ratios: The default 28/36 is the conventional benchmark. FHA loans allow higher ratios (commonly up to 31/43 with compensating factors); conventional loans with strong credit and large down payments may qualify at 45% or higher back-end ratios with automated underwriting. The calculator allows these to be adjusted.
Why Lenders Also Consider Other Factors
DTI ratios are the primary affordability screen, but lenders evaluate several additional factors:
Credit score: A higher credit score unlocks lower interest rates (and often higher DTI allowances). At identical income and debt levels, a borrower with a 760 score typically qualifies for a meaningfully lower rate than one with a 680 score — which changes the maximum loan amount the payment can support.
Loan-to-value ratio (LTV): The down payment percentage affects whether PMI is required (typically for LTVs above 80% on conventional loans). PMI adds 0.5%–1.5% of the loan amount annually to the monthly payment, which reduces the loan amount the binding payment cap can support.
Reserves: Lenders commonly require two to six months of mortgage payments in liquid savings after closing, in addition to the down payment and closing costs.
Loan type: Conventional, FHA, VA, and USDA loans each have different DTI standards, down payment minimums, and insurance requirements. The DTI numbers in this guide reflect conventional lending guidelines.
What the 28/36 Rule Misses
The DTI calculation tells a borrower what they might qualify for — not necessarily what they can comfortably afford. Several real-world factors are worth considering alongside the output:
Property taxes and insurance vary by location. The 1.25% annual rule of thumb used in the calculation is a national average. Taxes in high-tax states (New Jersey, Illinois, Texas) can run 2%–3% or more of home value annually; in low-tax states, they may be below 0.5%. If the target market has unusually high or low property taxes, the actual PITI will differ from the estimate.
Maintenance and repairs. A general rule of thumb is to budget 1%–2% of home value annually for maintenance. This is not a debt payment (and does not appear in DTI calculations), but it is a real ongoing cost of ownership that affects how much of the maximum qualifying price is genuinely sustainable.
HOA dues. If buying in a community with a homeowners association, monthly HOA dues are included in the front-end DTI. The calculator currently excludes HOA dues from its estimate; if applicable, add the monthly HOA payment to the estimated PITI to verify the ratio is still within bounds.
Frequently Asked Questions
What’s the difference between pre-approval and this calculator’s output? Pre-approval is a lender’s formal statement, based on verified income, credit, assets, and debts, that they will lend up to a specific amount under specific conditions. This calculator uses the same 28/36 framework but does not verify inputs or apply lender-specific overlays. The calculator output is a planning guide; pre-approval from an actual lender is required before making an offer on a home.
Does a higher down payment increase the home I can afford? Yes, directly: a larger down payment means the loan amount can be smaller for the same home price, or the same loan amount can support a higher home price. The payment-capacity limit (from DTI) is fixed by income; a larger down payment allows that payment to service a larger total purchase. Additionally, a down payment of 20% or more on a conventional loan eliminates PMI, which frees up additional payment capacity.
Why does my actual mortgage payment differ from the calculator’s estimate? The main sources of divergence are: (1) actual property taxes and insurance that differ from the 1.25% rule-of-thumb estimate, (2) PMI if the down payment is below 20%, (3) HOA dues if applicable, and (4) the exact lender rate at the time of closing, which may differ from the rate assumed in the calculation.
Does refinancing affect affordability? A refinance replaces the existing mortgage with a new loan, typically to change the rate or term. If rates have fallen, refinancing can reduce the monthly payment on the same loan balance — effectively increasing the home-price ceiling for the same payment capacity. The calculator is designed for purchase decisions; for refinance analysis, the mortgage calculator provides amortization and payment comparisons at different rates.