To calculate mortgage affordability, apply the 28/36 debt-to-income rule to your gross monthly income and existing debts: your maximum housing payment is the smaller of 28% of your gross income and 36% of your gross income minus your other monthly debts, then run that payment through inverse loan amortization at your expected interest rate and term to find the largest loan you can carry, and add your down payment to reveal the top home price in your range. The result is a realistic ceiling on what you can borrow, not the maximum a lender might technically approve.
A free Home Affordability Calculator runs this entire calculation in your browser. You type in your income, debts, down payment, expected interest rate, and loan term, and the tool returns the affordable home price, the affordable loan amount, and the maximum monthly housing payment. Nothing leaves your device, so you can adjust the inputs as many times as you want while exploring listings or trade-offs.
What "Mortgage Affordability" Actually Means
Mortgage affordability is the price of the home you can realistically carry, given what you earn, what you already owe, and how much cash you can put down on closing day. It is different from "how much can I borrow" — a loan amount you qualify for at the bank is the upper bound set by underwriting rules, while affordability is the price you can comfortably service month after month without stretching your budget past the breaking point.
Most lenders anchor affordability to a debt-to-income (DTI) ratio, which compares your monthly debt payments to your gross (pre-tax) monthly income. The two most common benchmarks are the front-end ratio (housing cost as a share of income) and the back-end ratio (total monthly debt as a share of income). When a lender refers to the 28/36 rule, they are pointing at these two ceilings: 28% on the front end, 36% on the back end.
The 28/36 Rule Behind the Numbers
The 28/36 rule is a guideline, not a law, but it is the most widely cited starting point for buyer budgets. According to the Wikipedia overview of debt-to-income ratios, the front-end ratio of 28% sets how much of your gross income should go to housing, while the back-end ratio of 36% caps total monthly debt including the mortgage. The Consumer Financial Protection Bureau also describes how lenders use your DTI to size a mortgage when reviewing an application.
Your maximum housing payment under the rule is the smaller of two values:
- 0.28 × your gross monthly income (the front-end ceiling)
- 0.36 × your gross monthly income − your other monthly debts (the back-end ceiling)
When your other debts are low — say just a small car payment — the 28% front-end ratio usually binds, and you get the full 28% for housing. As debts climb, the back-end ratio pulls the number down. If your existing debts are already at or above 36% of income, the formula clamps to zero, which is the calculator's signal that you need to pay down debt before taking on a mortgage.
How to Calculate Mortgage Affordability
The Home Affordability Calculator follows the 28/36 rule and runs the inverse loan math for you. To get an accurate result, work through these three inputs in order.
- Enter your gross monthly income and your recurring monthly debts. Type in your pre-tax income and pick whether the figure is monthly or annual. Then add up your other monthly obligations — car loans, student loans, credit-card minimums — and enter the total. These two numbers drive the entire DTI calculation.
- Enter your cash down payment, expected interest rate, and loan term. The down payment is the cash you will bring to closing. The interest rate is your best estimate of what you will be quoted. The loan term is typically 15 or 30 years. These three inputs determine how much loan the maximum monthly payment can support.
- Read the affordable home price, affordable loan amount, and maximum monthly housing payment. The calculator shows all three outputs side by side, recalculated on every change. The home price is the loan plus the down payment; the loan is what the inverse amortization produced; the monthly payment is the ceiling the 28/36 rule set.
Reading the Three Output Numbers
Each output answers a different question, and the most useful one depends on where you are in the search.
| Output | What it tells you | When to use it |
|---|---|---|
| Maximum monthly housing payment | The DTI ceiling the 28/36 rule allows for principal and interest | Comparing against a listing's projected payment |
| Affordable loan amount | The largest loan your maximum payment supports at your rate and term | Pre-qualification conversations with a lender |
| Affordable home price | The loan plus your down payment — the realistic top of your search | Filtering listings and setting a target budget |
Notice the ordering: the payment comes first because the 28/36 rule sets it; the loan comes second because the rate and term turn that payment into principal; the home price comes last because it layers your cash on top. Walking through the calculator in that order is the fastest way to understand where each ceiling lives.
A Worked Example
To make the formula concrete, take a buyer with $6,000 in gross monthly income, $400 in other monthly debts, a $40,000 down payment, a 6.5% expected rate, and a 30-year term.
- Compute the maximum monthly payment. The front-end cap is 0.28 × $6,000 = $1,680. The back-end cap is 0.36 × $6,000 − $400 = $1,760. The maximum housing payment is the smaller of the two: $1,680.
- Run inverse amortization. With r = 0.065 / 12 ≈ 0.005417 and n = 30 × 12 = 360 months, the maximum loan is $1,680 × ((1.005417)^360 − 1) / (0.005417 × (1.005417)^360) ≈ $1,680 × 158.2 ≈ $265,800.
- Add the down payment. Affordable home price ≈ $265,800 + $40,000 ≈ $305,800.
The three numbers — $1,680 payment ceiling, $265,800 loan ceiling, and $305,800 price ceiling — are exactly what the calculator returns for these inputs.
How the Estimate Differs From a Mortgage Calculator
A standard mortgage calculator starts with a loan amount and computes the monthly payment. The Home Affordability Calculator does the opposite: it starts with your income and debts, applies the 28/36 rule to find the largest payment you can carry, and then runs inverse amortization to back out the loan amount and home price that payment supports.
| Calculator | Starting input | Direction of math | Typical use |
|---|---|---|---|
| Home Affordability Calculator | Income, debts, down payment | Income → payment → loan → home price | Setting a realistic budget before you shop |
| Mortgage Calculator | Loan amount, rate, term | Loan → monthly payment | Pricing a specific loan you are considering |
For a complete picture, run both. The affordability tool sets your ceiling; the mortgage calculator lets you model the loan terms inside that ceiling.
How Down Payment, Rate, and Term Move Your Budget
Because every output is recomputed instantly, the calculator is the fastest way to compare trade-offs.
- Larger down payment. A bigger down payment adds directly to the home price without changing your monthly payment ceiling, because the loan shrinks while the payment stays the same. Doubling your down payment roughly doubles the price expansion, all else equal.
- Lower interest rate. A lower rate lets the same monthly payment support a larger loan, which means a larger affordable home price. The size of the jump depends on the rate change and the term — a one-point drop on a 30-year loan moves the budget more than the same drop on a 15-year loan.
- Shorter term (15 vs. 30 years). A shorter term shrinks the maximum loan because the payment has to amortize the balance over fewer months. A 15-year term usually produces a smaller affordable home price than a 30-year term at the same rate, but with substantially less interest paid over the life of the loan.
- Paying down other debts. Reducing car loans, student loans, or credit-card minimums raises the back-end ceiling directly, which raises the maximum housing payment and therefore the home price. This is often the single most powerful lever short of increasing income.
Use the calculator to test each lever in isolation first, then combine the realistic ones. The shape of the changes matters more than any single round-number estimate, and the tool recomputes as you adjust.
What the Estimate Leaves Out
The Home Affordability Calculator covers loan principal and interest only. Property taxes, homeowners insurance, HOA dues, and private mortgage insurance (PMI) when your down payment is below 20% all add to your true monthly cost. None of these are priced into the output, so the home price the calculator returns is an upper bound on what you can comfortably carry under the 28/36 rule, not the full monthly outlay.
The 28/36 rule itself is a guideline, not a guarantee. Real underwriting also weighs your credit score, cash reserves, employment history, and the specific loan program. FHA, VA, and conventional loans each use different limits, and some lenders stretch ratios higher for strong borrowers. Treat the calculator's output as a starting benchmark for your search, then confirm the actual numbers with a licensed mortgage professional before you make an offer.
For a broader walkthrough that ties the same inputs to the 28/36 rule in plain English, see how to calculate home affordability based on income.