Determining your home affordability is the first critical step in the home buying process. This calculator uses the standard debt-to-income (DTI) ratios employed by most lenders to estimate your purchasing power. Understanding how these numbers work can help you budget effectively and avoid mortgage rejection.
The 28/36 Rule Explained
Lenders typically use two ratios to decide how much they will lend you:
The Front-End Ratio (28%): This rule states that your total monthly housing costs (principal, interest, taxes, and insurance) should not exceed 28% of your gross monthly income.
The Back-End Ratio (36%): This rule states that your total monthly debt payments (housing costs plus student loans, car payments, credit cards, etc.) should not exceed 36% of your gross monthly income.
Our calculator checks both ratios and uses the lower of the two amounts to ensure you stay within a conservative financial safety zone.
Factors That Impact Your Affordability
Several variables can significantly change your maximum home price:
Interest Rates: Even a 1% increase in rates can reduce your buying power by tens of thousands of dollars.
Down Payment: A larger down payment reduces the loan amount needed, allowing you to buy a more expensive home for the same monthly payment.
Existing Debts: High monthly debts (like car loans) eat into your back-end ratio, directly reducing the mortgage amount you qualify for.
Property Taxes & Insurance: In high-tax areas, a significant portion of your monthly payment goes to taxes, leaving less room for the mortgage principal.
Tips for Increasing Your Budget
If the result isn't what you hoped for, consider paying off high-interest credit card debt to lower your monthly obligations. Alternatively, saving for a larger down payment not only increases your budget but may also help you avoid Private Mortgage Insurance (PMI).
function calculateAffordability() {
var annualIncome = parseFloat(document.getElementById('annualIncome').value);
var monthlyDebts = parseFloat(document.getElementById('monthlyDebts').value) || 0;
var downPayment = parseFloat(document.getElementById('downPayment').value) || 0;
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTerm = parseFloat(document.getElementById('loanTerm').value);
var monthlyTaxIns = parseFloat(document.getElementById('monthlyTaxIns').value) || 0;
// Validate inputs
if (isNaN(annualIncome) || isNaN(interestRate) || isNaN(loanTerm) || annualIncome <= 0 || loanTerm <= 0) {
alert("Please enter valid positive numbers for Income, Interest Rate, and Loan Term.");
return;
}
var monthlyIncome = annualIncome / 12;
var r = (interestRate / 100) / 12;
var n = loanTerm * 12;
// Calculate Limits
// Front-End: 28% of Income for Housing (PITI)
var frontEndLimit = monthlyIncome * 0.28;
// Back-End: 36% of Income for Total Debts (Housing + Other Debts)
var backEndLimit = (monthlyIncome * 0.36) – monthlyDebts;
// Determine the limiting monthly payment (Total Housing Expense)
var maxTotalHousingPayment = Math.min(frontEndLimit, backEndLimit);
// Identify which ratio limited the loan
var limitingFactor = (frontEndLimit < backEndLimit) ? "Front-End Ratio (Income)" : "Back-End Ratio (Debts)";
// Calculate Max P&I (Principal & Interest) available
// Max P&I = Max Total Housing – Taxes/Insurance
var maxPI = maxTotalHousingPayment – monthlyTaxIns;
var maxLoanAmount = 0;
var maxHomePrice = 0;
if (maxPI <= 0) {
maxPI = 0;
maxLoanAmount = 0;
maxHomePrice = downPayment;
maxTotalHousingPayment = monthlyTaxIns; // minimal cost
document.getElementById('resLimitingFactor').innerText = "High Expenses / Low Income";
} else {
// Calculate Loan Amount based on Max P&I
// Formula: PV = PMT * (1 – (1 + r)^-n) / r
// Note: If interest is 0, PV = PMT * n
if (interestRate === 0) {
maxLoanAmount = maxPI * n;
} else {
var discountFactor = (1 – Math.pow(1 + r, -n)) / r;
maxLoanAmount = maxPI * discountFactor;
}
maxHomePrice = maxLoanAmount + downPayment;
document.getElementById('resLimitingFactor').innerText = limitingFactor;
}
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
// Update DOM
document.getElementById('resMaxPrice').innerText = formatter.format(maxHomePrice);
document.getElementById('resMaxPI').innerText = formatter.format(maxPI);
document.getElementById('resTotalPayment').innerText = formatter.format(Math.max(0, maxTotalHousingPayment));
document.getElementById('ma-result').style.display = 'block';
}