Buying a home is one of the biggest financial decisions you'll make. Understanding how much you can realistically afford for a mortgage is a crucial first step. This calculator helps you estimate your maximum affordable mortgage amount based on your income, debts, and current interest rates. It's important to remember that this is an estimate, and your lender will perform a more detailed analysis.
function calculateMortgageAffordability() {
var grossMonthlyIncome = parseFloat(document.getElementById("grossMonthlyIncome").value);
var existingMonthlyDebt = parseFloat(document.getElementById("existingMonthlyDebt").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTermYears").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(grossMonthlyIncome) || grossMonthlyIncome <= 0 ||
isNaN(existingMonthlyDebt) || existingMonthlyDebt < 0 ||
isNaN(downPayment) || downPayment < 0 ||
isNaN(interestRate) || interestRate <= 0 ||
isNaN(loanTermYears) || loanTermYears <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// PITI (Principal, Interest, Taxes, Insurance) is often limited to 28-36% of gross monthly income.
// We'll use a conservative Debt-to-Income (DTI) ratio of 36% for total debt,
// and a housing-specific ratio of 28%.
var maxHousingPayment = grossMonthlyIncome * 0.28; // Housing PITI
var maxTotalDebtPayment = grossMonthlyIncome * 0.36;
var maxMortgagePayment = maxTotalDebtPayment – existingMonthlyDebt;
// Ensure maxMortgagePayment is not negative
if (maxMortgagePayment < 0) {
maxMortgagePayment = 0;
}
// Use the lower of the two calculated maximum payments
var affordableMonthlyPITI = Math.min(maxHousingPayment, maxMortgagePayment);
// Estimate Taxes and Insurance (often 1-1.5% of home value annually, but can vary greatly)
// For simplicity, let's estimate taxes and insurance as a fixed percentage of the loan amount
// This is a simplification; actual PITI will be calculated by lender.
// We'll use this to back-calculate the loan amount.
var estimatedAnnualTaxesInsuranceRate = 0.012; // 1.2% of loan value annually
var estimatedMonthlyTaxesInsurance = 0; // Will be calculated based on estimated loan amount
// We need to find the loan amount (L) such that:
// AffordableMonthlyPITI = MonthlyPrincipalInterest + MonthlyTaxesInsurance
// AffordableMonthlyPITI = P * [i(1+i)^n] / [(1+i)^n – 1] + L * (estimatedAnnualTaxesInsuranceRate / 12)
// Where P is the loan amount, i is the monthly interest rate, n is the number of months.
var monthlyInterestRate = (interestRate / 100) / 12;
var loanTermMonths = loanTermYears * 12;
var maxLoanAmount = 0;
// Iterate to find an approximate loan amount, as taxes/insurance depend on loan amount itself.
// This is an iterative approach. A more precise analytical solution exists but is more complex.
// We'll start with an estimated loan amount and refine it.
var currentLoanEstimate = affordableMonthlyPITI * loanTermMonths; // Rough initial guess
for (var i = 0; i < 100; i++) { // Iterate up to 100 times for approximation
var estimatedMonthlyTaxesInsurance = (currentLoanEstimate * estimatedAnnualTaxesInsuranceRate) / 12;
var affordableMonthlyPI = affordableMonthlyPITI – estimatedMonthlyTaxesInsurance;
if (affordableMonthlyPI 0) {
calculatedLoanAmount = affordableMonthlyPI * (Math.pow(1 + monthlyInterestRate, loanTermMonths) – 1) / (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, loanTermMonths));
} else { // Handle 0% interest rate case (though unlikely for mortgages)
calculatedLoanAmount = affordableMonthlyPI * loanTermMonths;
}
if (Math.abs(calculatedLoanAmount – currentLoanEstimate) < 10) { // Convergence
currentLoanEstimate = calculatedLoanAmount;
break;
}
currentLoanEstimate = calculatedLoanAmount;
}
maxLoanAmount = currentLoanEstimate;
var maxHomePrice = maxLoanAmount + downPayment;
// Format results
var formattedMaxLoanAmount = maxLoanAmount.toLocaleString(undefined, { minimumFractionDigits: 2, maximumFractionDigits: 2 });
var formattedMaxHomePrice = maxHomePrice.toLocaleString(undefined, { minimumFractionDigits: 2, maximumFractionDigits: 2 });
var formattedAffordableMonthlyPITI = affordableMonthlyPITI.toLocaleString(undefined, { minimumFractionDigits: 2, maximumFractionDigits: 2 });
resultDiv.innerHTML = `
Your Estimated Affordability
Based on your inputs, your estimated maximum affordable mortgage loan amount is: $${formattedMaxLoanAmount}
Considering your down payment of $${downPayment.toLocaleString(undefined, { minimumFractionDigits: 2, maximumFractionDigits: 2 })}, your estimated maximum home price is: $${formattedMaxHomePrice}
Your estimated maximum monthly payment (Principal, Interest, Taxes, Insurance – PITI) is approximately: $${formattedAffordableMonthlyPITI}
Note: This is an estimate. Actual loan approval and amounts depend on lender's underwriting, credit score, market conditions, and specific property taxes/insurance costs.
`;
}
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font-family: sans-serif;
padding: 20px;
border: 1px solid #ddd;
border-radius: 8px;
max-width: 600px;
margin: 20px auto;
background-color: #f9f9f9;
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text-align: center;
margin-bottom: 20px;
color: #333;
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margin-bottom: 5px;
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transition: background-color 0.3s ease;
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background-color: #0056b3;
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margin-top: 25px;
padding: 15px;
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background-color: #fff;
border-radius: 4px;
text-align: center;
}
.calculator-result h3 {
margin-top: 0;
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}
.calculator-result p {
margin-bottom: 10px;
line-height: 1.5;
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font-size: 0.9em;
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display: block;
margin-top: 15px;
}