Determining your home buying power is the first critical step in the real estate journey. This calculator uses the standard debt-to-income (DTI) ratios utilized by most mortgage lenders to estimate the maximum loan size you qualify for.
Understanding the Calculation Logic
Lenders primarily use two ratios to decide your borrowing limit. We calculate both and use the lower number to ensure a realistic estimate:
Front-End Ratio (28%): Lenders prefer your housing costs (Principal, Interest, Taxes, and Insurance – PITI) not to exceed 28% of your gross monthly income.
Back-End Ratio (36%): Your total monthly debt obligations (housing costs + credit cards, student loans, car payments) should generally not exceed 36% of your gross monthly income.
Key Factors Affecting Your Results
While income is important, your Interest Rate and Monthly Debts play massive roles. A higher interest rate reduces your buying power significantly because more of your monthly payment goes toward interest rather than principal. Similarly, high monthly debts (like a large car payment) reduce the "Back-End" allowance, directly lowering the mortgage amount you can carry.
Improving Your Affordability
If the result is lower than expected, consider: increasing your down payment to reduce the loan amount, paying off smaller debts to free up monthly cash flow, or shopping for a lower interest rate.
function calculateHouseAffordability() {
// 1. Get Inputs
var annualIncome = parseFloat(document.getElementById('acAnnualIncome').value);
var monthlyDebt = parseFloat(document.getElementById('acMonthlyDebts').value);
var downPayment = parseFloat(document.getElementById('acDownPayment').value);
var interestRate = parseFloat(document.getElementById('acInterestRate').value);
var loanTermYears = parseFloat(document.getElementById('acLoanTerm').value);
var annualTax = parseFloat(document.getElementById('acPropertyTax').value);
var annualInsurance = parseFloat(document.getElementById('acHomeInsurance').value);
// Validation / Defaults
if (isNaN(annualIncome)) annualIncome = 0;
if (isNaN(monthlyDebt)) monthlyDebt = 0;
if (isNaN(downPayment)) downPayment = 0;
if (isNaN(interestRate)) interestRate = 6.5; // Default fallback
if (isNaN(loanTermYears)) loanTermYears = 30;
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
// 2. Calculate Monthly Basis
var monthlyIncome = annualIncome / 12;
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
// 3. Determine Max Monthly PITI (Principal, Interest, Tax, Insurance)
// Rule 1: Front-End Ratio (28% of Income)
var maxPITI_Front = monthlyIncome * 0.28;
// Rule 2: Back-End Ratio (36% of Income – Debts)
var maxPITI_Back = (monthlyIncome * 0.36) – monthlyDebt;
// Use the stricter (lower) limit
var maxAllowablePITI = Math.min(maxPITI_Front, maxPITI_Back);
var limitFactor = (maxPITI_Front < maxPITI_Back) ? "Income (Front-End)" : "Debt (Back-End)";
// If debts are too high, result could be negative
if (maxAllowablePITI < 0) maxAllowablePITI = 0;
// 4. Isolate Principal & Interest (P&I) availability
var availableForPI = maxAllowablePITI – monthlyTax – monthlyInsurance;
if (availableForPI 0 && interestRate > 0) {
var r = (interestRate / 100) / 12;
var n = loanTermYears * 12;
maxLoanAmount = availableForPI * (1 – Math.pow(1 + r, -n)) / r;
} else if (availableForPI > 0 && interestRate === 0) {
// Edge case: 0% interest
maxLoanAmount = availableForPI * (loanTermYears * 12);
}
// 6. Calculate Max Home Price
var maxHomePrice = maxLoanAmount + downPayment;
// 7. Format Output
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
document.getElementById('acMaxPriceResult').innerHTML = formatter.format(maxHomePrice);
document.getElementById('acMaxMonthlyResult').innerHTML = formatter.format(maxAllowablePITI);
document.getElementById('acMaxLoanResult').innerHTML = formatter.format(maxLoanAmount);
document.getElementById('acLimitFactor').innerHTML = limitFactor;
// Show Results
document.getElementById('ac-results').style.display = 'block';
}