Advanced Mortgage Affordability Calculator
Calculate the maximum home purchase price you can afford based on your income, existing debts, down payment, interest rate, and local property taxes. Use this before starting your home search to set a realistic budget.
Last updated: May 2026
About this calculator
Mortgage affordability is governed by the debt-to-income (DTI) ratio — the percentage of gross monthly income that can go toward all debt payments. Lenders typically cap total DTI at 36–43% depending on loan program. The maximum allowable monthly housing payment is: P_avail = (grossIncome / 12 × DTI%) − monthlyDebts. This budget must cover principal & interest, property taxes, and (when down payment is below 20%) private mortgage insurance — the PITI components most directly tied to the loan size. The closed-form solution uses the relation home price H = loan L / (1 − down/100) to substitute and back-solve algebraically: P_avail = L × payment_factor + H × tax_monthly_rate + L × pmi_monthly_rate, which becomes P_avail = L × [payment_factor + (tax_rate/12) / (1−down/100) + pmi_term], and dividing by (1−down/100) gives home price directly: H = P_avail / [(1−down/100) × payment_factor + (tax_rate/12) + (1−down/100) × pmi_term]. payment_factor is the standard amortization factor r(1+r)ⁿ / ((1+r)ⁿ−1) with n=360. PMI is assumed at 0.5% annually on the loan when down payment is below 20% (conventional convention); homeowners insurance is not included — add roughly $100–200/month yourself if you want a fuller PITI budget.
How to use
Example: $90,000 gross annual income, $400/month in existing debts, 10% down payment, 6.5% interest rate, 1.2% annual property tax rate, 43% DTI (FHA Maximum). Step 1 — P_avail = $90,000/12 × 0.43 − $400 = $3,225 − $400 = $2,825. Step 2 — monthly rate r = 0.065/12 ≈ 0.005417; (1+r)³⁶⁰ ≈ 6.991; payment_factor = (0.005417 × 6.991) / (6.991 − 1) ≈ 0.006322. Step 3 — LTV ratio (1 − down/100) = 0.9; tax_monthly_rate = 0.012/12 = 0.001; pmi_term = 0.9 × 0.005/12 ≈ 0.000375 (since down < 20%, PMI applies). Step 4 — denominator = 0.9 × 0.006322 + 0.001 + 0.000375 = 0.005690 + 0.001 + 0.000375 ≈ 0.007065. Step 5 — Home price = $2,825 / 0.007065 ≈ $399,860. Backing out the loan: $399,860 × 0.9 = $359,874; monthly P&I = $359,874 × 0.006322 ≈ $2,275; monthly tax = $399,860 × 0.001 = $400; monthly PMI = $359,874 × 0.000417 ≈ $150; total = $2,825 ✓. Raise the down payment to 20% (PMI drops out and LTV falls from 0.9 to 0.8) and the affordable home price rises to roughly $466,000 for the same $2,825 budget — a 17% increase from removing PMI and freeing up more P&I capacity.
Frequently asked questions
What debt-to-income ratio do mortgage lenders require for approval?
Most conventional lenders prefer a total debt-to-income ratio of 43% or below, though some programs allow up to 50% for borrowers with strong credit and large down payments. The front-end DTI (housing costs only) is ideally kept under 28–31%. FHA loans typically allow a back-end DTI up to 43–57% depending on compensating factors such as cash reserves or high credit scores. Using a conservative DTI like 36–40% in your affordability calculation gives you a comfortable buffer and lowers the risk of being house-poor.
How does the down payment percentage affect how much house I can afford?
A larger down payment directly increases the home price you can afford on the same monthly budget, because a smaller loan is needed to purchase the home. It also eliminates or reduces PMI (private mortgage insurance), which adds 0.2–1% of the loan balance annually to your costs. Additionally, putting down 20% or more typically qualifies you for better interest rates. Every extra percentage point of down payment effectively converts dollar-for-dollar into additional purchasing power, since the loan amount shrinks and more of your monthly budget goes toward a higher-priced property.
Why do property taxes affect mortgage affordability calculations?
Property taxes are included in your monthly housing payment (as part of PITI — principal, interest, taxes, insurance), and lenders count them against your DTI limit. High-tax states like New Jersey or Illinois can add $500–$1,500 or more per month to housing costs on a $400,000 home, significantly reducing how large a mortgage you qualify for. For example, a 2.5% property tax rate versus a 0.5% rate on the same home can reduce your affordable loan amount by $50,000–$100,000 on a moderate income, even if your mortgage payment would be identical.