The 28/36 debt-to-income rule caps housing costs at 28% of gross monthly income and total recurring debt at 36%, and the Home Affordability Calculator uses those two ceilings to translate your income, debts, down payment, rate, and term into a maximum affordable home price. For a buyer with $80,000 in gross annual income, $300 in monthly car loan payments, and a $20,000 down payment quoted at 7% over 30 years, the calculator returns roughly $1,866.67 as the maximum monthly housing payment, about $280,600 as the maximum loan amount, and roughly $300,600 as the top of the realistic price range — a number that lands where it does precisely because the front-end 28% ratio binds before the back-end 36% ratio does. Walking through the example is the clearest way to see how each input feeds the result, why your existing debts can quietly shrink your housing budget, and where the formula has natural limits you should treat as a starting benchmark rather than a final lending decision.
How the 28/36 Rule Sets Your Housing Budget
The Home Affordability Calculator starts where most buyer questions start: "what monthly payment can I actually carry?" It answers that with the 28/36 debt-to-income guideline, the same front-end and back-end ratio framework that lenders reference when sizing loans (Consumer Financial Protection Bureau, "What is a debt-to-income ratio?"; Wikipedia: Debt-to-income ratio).
The rule sets two ceilings. The front-end ratio limits housing costs — principal and interest on your future mortgage — to no more than 28% of your gross monthly income. The back-end ratio limits all recurring monthly debt — your future mortgage plus car loans, student loans, and minimum credit-card payments — to no more than 36% of gross income. Your maximum housing budget is the smaller of the two numbers:
- Front-end cap = 0.28 × gross monthly income
- Back-end cap = (0.36 × gross monthly income) − existing monthly debts
| Ratio | Ceiling | Formula | What It Counts |
|---|---|---|---|
| Front-end | 28% of gross monthly income | 0.28 × income | Mortgage principal and interest only |
| Back-end | 36% of gross monthly income | 0.36 × income − other monthly debts | Mortgage plus car loans, student loans, minimum credit-card payments |
When your other debts are low, the front-end ratio is usually the binding number. When debts are high — multiple car loans, large student loan balances, or significant credit-card minimums — the back-end ratio pulls the maximum down, and in extreme cases it can drive the housing budget to zero. The calculator clamps the result at zero so that no negative payment ever appears in the output.
That maximum monthly payment is then run backward through the standard mortgage formula (the annuity present-value formula) at the rate and term you enter, which converts a monthly payment ceiling into a maximum loan amount. Add the cash down payment, and you get the affordable home price. Adjusting any input recalculates the entire stack in seconds, so you can compare a 15-year versus 30-year term, or see what paying off a car loan would unlock, by changing one field at a time.
A Worked Example With $80,000 Income and $20,000 Down
To see the formula in action, plug these inputs into the Home Affordability Calculator:
- Gross annual income: $80,000 (entered as annual)
- Monthly recurring debts: $300 (one car loan)
- Down payment: $20,000
- Annual interest rate: 7%
- Loan term: 30 years
Step 1 — convert income to monthly. $80,000 ÷ 12 = $6,666.67 per month.
Step 2 — apply the front-end cap. 0.28 × $6,666.67 = $1,866.67.
Step 3 — apply the back-end cap. 0.36 × $6,666.67 = $2,400.00; subtract the $300 in monthly debts to get $2,100.00.
Step 4 — take the smaller of the two. $1,866.67 is less than $2,100.00, so the front-end ratio binds and the maximum monthly housing payment is $1,866.67. The back-end ratio would have allowed a slightly larger payment, but the rule uses the more conservative number.
Step 5 — run inverse amortization. With r = 0.07 ÷ 12 = 0.005833 and n = 30 × 12 = 360 months, the annuity present-value factor is (1 − (1 + r)−n) ÷ r. Computing (1.005833)360 ≈ 8.1165, so the factor equals (1 − 1 ÷ 8.1165) ÷ 0.005833 ≈ (1 − 0.1232) ÷ 0.005833 ≈ 0.8768 ÷ 0.005833 ≈ 150.31. Multiply by the monthly payment: $1,866.67 × 150.31 ≈ $280,579.
Step 6 — add the down payment. $280,579 + $20,000 = $300,579, which rounds to roughly $300,600 as the top of the realistic price range.
So the example resolves to three outputs: a maximum monthly housing payment of $1,866.67, a maximum loan of about $280,600, and an affordable home price of about $300,600. The same person with the same $80,000 income but no other debts would land on the same numbers — the back-end cap of $2,400 is unused slack in this example because the front-end cap binds first. To see what a similar buyer would pay on a known loan amount instead of working backward from income, run the numbers in the mortgage calculator.
How to Use the Home Affordability Calculator
The tool follows a deliberate input order: income and debts first, then down payment, rate, and term, then read the three outputs.
- Enter your gross income (annual or monthly) and add up your total recurring monthly debt payments — car loans, student loans, and credit-card minimums. These two numbers drive the 28/36 calculation.
- Enter the cash down payment you have available, the annual interest rate you expect to be quoted (or the current national average if you are planning ahead), and the loan term in years — 15, 20, or 30 are the standard choices.
- Read the three outputs: the affordable home price, the affordable loan amount, and the maximum monthly housing payment. All three recalculate as you change any field, so the comparison between a 15-year and a 30-year term is a single click apart.
Everything runs in your browser, which means nothing you type is uploaded, stored, or shared with a lender or a third party. The result is a planning benchmark, not a preapproval.
What the Result Leaves Out of Your Real Budget
The calculator's $1,866.67 ceiling and roughly $300,600 home price describe principal and interest only. Your real monthly housing cost on the same home would also include property taxes, homeowners insurance, and — if the down payment is below 20% on a conventional loan — private mortgage insurance. HOA dues apply in many condo and planned-community developments. Each of those line items is independent of the 28/36 rule and can quietly shrink the home price you can comfortably carry.
Beyond the monthly extras, the 28/36 rule is a guideline rather than a guarantee. Actual underwriting also weighs your credit score, cash reserves after closing, employment history, and the specific loan program — FHA, VA, and conventional loans each use different limits, and some lenders will stretch the ratios higher for strong borrowers. Treat the $300,600 figure as a starting benchmark for your search and your budget conversations, then confirm the actual numbers with a licensed mortgage professional before you make an offer on a specific property.
How to Stress-Test the Number Before You Shop
Because every input recalculates in the browser, the easiest way to turn this single example into a personal plan is to swap one field at a time and watch the three outputs move.
- Pay down the car loan. Drop the $300 monthly debt to $0 and the back-end cap rises from $2,100 to $2,400. In this example the front-end ratio still binds, so the home price barely moves, but the higher debt headroom gives you flexibility on a stronger offer.
- Raise the down payment. Increase from $20,000 to $40,000 and the affordable home price rises by roughly the same $20,000, because the down payment is added directly to the loan amount.
- Change the term. A 15-year term has a smaller annuity factor than the 30-year term, so the maximum loan shrinks; a 30-year term lets you stretch the same payment into a larger loan. Run the comparison in the Home Affordability Calculator to see the exact change.
- Compare scenarios side by side. For any of these comparisons, run the inputs in the calculator and record the resulting loan and price — the relationship between inputs and outputs is the same every time, but the magnitudes depend on your numbers.