How to Calculate Your Mortgage Affordability

Why Calculate Affordability Yourself

Mortgage lenders will often qualify you for a loan larger than is financially wise. Their calculation focuses on whether you can make the payment, not whether that payment leaves you room for savings, emergencies, and a life worth living. A bank's maximum is not your maximum — it's the most they'll lend, often stretching your budget to the point where you become "house-poor": owning a home you can barely afford, with little left for anything else.

Calculating your own affordability — what you can comfortably afford while still saving, handling emergencies, and enjoying life — is one of the most important steps before house hunting. It prevents the most common and costly home-buying mistake: buying more house than your finances can sustain, which leads to stress, depleted savings, and sometimes foreclosure if income drops.

The right affordability calculation considers your full financial picture: income, debts, down payment, all the costs of ownership (not just the mortgage), and your other financial goals. This guide walks through the proven 28/36 rule, the costs beyond the mortgage, a step-by-step calculation, and how to avoid the house-poor trap.

The 28/36 Rule Explained

The 28/36 rule is the most widely used guideline for mortgage affordability, and it's a strong starting framework.

The 28% rule (front-end ratio): your total monthly housing cost should not exceed 28% of your gross (pre-tax) monthly income. "Total housing cost" includes the mortgage principal and interest, property taxes, homeowners insurance, and HOA dues if any — often abbreviated as PITI (Principal, Interest, Taxes, Insurance) plus HOA. This ratio ensures your housing doesn't consume so much of your income that you can't save or handle other expenses.

The 36% rule (back-end ratio): your total monthly debt — housing plus all other debts (car loans, student loans, credit card minimums, personal loans, child support) — should not exceed 36% of gross monthly income. This ratio ensures your total debt burden is sustainable.

A worked example: with a gross monthly income of $7,000 ($84,000/year), the 28% rule caps total housing at $1,960/month, and the 36% rule caps total debt at $2,520/month. If you have $500/month in other debts (car, student loan), your maximum housing payment under the 36% rule is $2,520 - $500 = $2,020. The tighter of the two limits governs — here, the 28% rule ($1,960) is the binding constraint.

Why these ratios? They're derived from historical default data: borrowers whose housing and debt ratios exceed these levels default at higher rates. Lenders use them (often with some flexibility) because they predict sustainability. But they're guidelines, not guarantees — your specific situation (job stability, savings, other obligations) may warrant a more conservative limit.

A more conservative approach: many financial advisors suggest capping housing at 25% of take-home (after-tax) income rather than 28% of gross, to leave more room for savings and life. This is especially wise for households with variable income, high taxes, or significant other goals. Use our mortgage affordability calculator to apply both rules to your numbers.

The Costs Beyond the Mortgage

The mortgage payment is only part of the cost of owning a home. Many first-time buyers focus on the mortgage and underestimate the other costs, which can add hundreds per month and thousands per year. A complete affordability calculation includes all of them.

Property taxes — vary widely by location, often 0.5–2.5% of home value annually. On a $400,000 home in a 1.5% area, that's $6,000/year or $500/month — a significant cost the mortgage ignores. Check the tax rate for your area; it's a major factor in total housing cost.

Homeowners insurance — protects the home and your liability, typically $1,000–$3,000/year depending on location, coverage, and home value. Required by lenders. See our home insurance guide.

Private mortgage insurance (PMI) — required if your down payment is under 20%, adding $50–$200+/month depending on the loan. Drops off once you reach 20% equity. See our PMI calculator.

HOA dues — if your home is in a homeowners association, monthly dues can range from modest to substantial, and they can rise. Factor them into your housing cost.

Maintenance and repairs — a rule of thumb is 1–2% of the home's value annually for ongoing maintenance and repairs. On a $400,000 home, that's $4,000–$8,000/year ($330–$670/month). New homes cost less; older homes cost more. This is the cost most underestimated by first-time buyers — and the one that causes the most financial surprises.

Utilities — often higher in a larger home than an apartment: electricity, gas, water, trash, internet. Budget for the home's actual costs, not your current rental's.

Property tax and insurance increases — both tend to rise over time. Budget for increases, not just the first-year costs.

The total of these costs can approach or exceed the mortgage payment itself. A $2,000 mortgage can become a $3,000+ total monthly housing cost once taxes, insurance, PMI, and maintenance are included. Your affordability calculation must include all of them, or you'll underestimate the true cost and overbuy.

Step-by-Step Affordability Calculation

Here's a complete, conservative calculation of the home price you can comfortably afford.

1. Calculate your gross monthly income. Use your pre-tax income. For variable income, use a conservative average.

2. Apply the 28% rule. Multiply gross monthly income by 0.28 to get your maximum total monthly housing cost (PITI + HOA). For $7,000/month, that's $1,960.

3. Subtract non-mortgage housing costs. Estimate property taxes, insurance, PMI (if under 20% down), and HOA for the price range you're considering, and subtract them from your maximum to find the mortgage payment you can afford. If taxes + insurance + PMI = $600, your affordable mortgage payment is $1,960 - $600 = $1,360.

4. Apply the 36% rule. Multiply gross monthly income by 0.36, subtract your other monthly debts, and confirm your housing cost fits. For $7,000 with $500 in other debts: $2,520 - $500 = $2,020 max housing — consistent with the 28% rule's $1,960.

5. Convert the affordable mortgage payment to a loan amount. Using current rates and your chosen term, calculate the loan size that produces your affordable payment. A $1,360 payment at 6% over 30 years supports roughly a $226,000 loan. Use our loan payment calculator.

6. Add your down payment to get the affordable home price. With a $50,000 down payment, your affordable home price is $226,000 + $50,000 = $276,000.

7. Stress-test the result. Could you still afford the payment if rates rose 1–2% (if you have an ARM), if your income dropped 20%, or if a major repair hit in year one? If not, your affordability is too aggressive — reduce your target price.

8. Consider a more conservative cap. Many advisors suggest capping housing at 25% of take-home income for more breathing room. Calculate both and choose the more conservative figure that fits your goals.

This calculation produces a home price you can comfortably afford — with room for savings, emergencies, and life. It will almost always be lower than the bank's maximum, and that's the point.

How Down Payment Affects Affordability

Your down payment directly affects your affordability in several ways.

Larger down payment, lower payment. A larger down payment means a smaller loan, a lower monthly payment, and less total interest. It also avoids PMI (at 20%+ down), saving $50–$200+/month. A bigger down payment increases the home price you can afford for a given monthly payment.

Smaller down payment, higher costs. A smaller down payment means a larger loan, a higher payment, and PMI. It reduces the home price you can afford for a given payment. However, it lets you buy sooner with less saved — a trade-off between waiting to save more and buying now with PMI.

The 20% threshold. At 20% down, you avoid PMI and get the best rates — the most cost-effective down payment if you can reach it without depleting savings. Below 20%, you'll pay PMI until you reach 20% equity. Don't deplete your emergency fund to reach 20% — having reserves matters more than avoiding PMI.

Down payment assistance. Many states and localities offer down payment assistance programs for first-time and moderate-income buyers — grants, forgivable loans, or low-interest second mortgages. Check what's available in your area; it can make homeownership accessible with less saved.

The total upfront cost is down payment + closing costs (2–5% of the loan) + an emergency reserve. Don't buy without savings left after closing. See our first home buying guide for the full down payment picture.

Avoiding the House-Poor Trap

Being house-poor means your housing costs consume so much of your income that you can't save, handle emergencies, or enjoy life. It's a common and stressful condition, usually caused by buying at the bank's maximum rather than your own. Here's how to avoid it.

Buy below your maximum. The bank's maximum is not your target — it's the ceiling. Aim for 10–20% below what you qualify for, leaving room for savings, emergencies, and life. A smaller, more affordable home is far better than a larger one that stresses your finances.

Include all costs in your calculation. The house-poor trap usually comes from ignoring taxes, insurance, PMI, and maintenance. Include all of them in your affordability calculation, and budget 1–2% of home value annually for maintenance.

Maintain an emergency fund. A home will need repairs — often inconveniently. Keep 3–6 months of expenses in savings (including the new housing cost) so a broken furnace or roof isn't a crisis. See our emergency fund guide.

Protect your other goals. Don't let housing crowd out retirement savings, emergency fund building, or other financial goals. If buying a home means stopping retirement contributions, you're buying too much house. Your affordability should leave room for all your priorities.

Plan for rate changes (if ARM). If you choose an adjustable-rate mortgage, ensure you can afford the payment at the maximum possible rate, not just the introductory rate. ARMs can reset higher and trap house-poor owners.

Consider future life changes. Will you have children, change jobs, or face income changes? Build in margin for the life you'll actually have, not just today's numbers.

For official guidance, the Consumer Financial Protection Bureau provides detailed, up-to-date information.

You can verify current figures directly with the Fannie Mae.

The HUD is a reliable source for the latest rules and limits.

The Bottom Line

Calculating your mortgage affordability yourself — rather than accepting the bank's maximum — is one of the most important steps in home buying. Use the 28/36 rule as a framework, include all the costs of ownership (taxes, insurance, PMI, maintenance, utilities), calculate a home price you can comfortably afford with room for savings and life, and buy below your maximum to avoid the house-poor trap. The home you can comfortably afford is almost always smaller than the one the bank will finance — and it's the one that lets you build wealth and enjoy life rather than stress over payments. Use our mortgage affordability calculator to run your numbers, and see our first home buying guide and lender guide for the next steps.