Understanding your potential monthly mortgage payment is a crucial step in the home-buying process. Our Mortgage Calculator helps you estimate your monthly housing costs by factoring in the loan principal, interest, taxes, and insurance (often referred to as PITI).
To get the most accurate result, simply enter the home price, your planned down payment, the interest rate offered by your lender, and the loan term (usually 15 or 30 years). Don't forget to include estimates for property taxes, homeowner's insurance, and HOA fees if applicable.
Understanding the Mortgage Formula
The core calculation for your monthly principal and interest payment uses the standard amortization formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
M = Total monthly payment
P = Principal loan amount (Home Price minus Down Payment)
i = Monthly interest rate (Annual rate divided by 12)
n = Number of payments (Loan term in years multiplied by 12)
Factors Affecting Your Monthly Payment
While the interest rate and home price are the biggest drivers of your monthly payment, other factors play a significant role:
Down Payment: A larger down payment reduces your principal loan amount and can eliminate the need for Private Mortgage Insurance (PMI), significantly lowering your monthly costs.
Property Taxes: These vary wildly by location. Local government tax rates can add hundreds of dollars to your monthly bill.
Homeowner's Insurance: This protects your property against damage. Costs depend on coverage limits, location, and the home's condition.
HOA Fees: If you buy a condo or a home in a planned community, Homeowners Association fees cover shared amenities and maintenance.
Why Use a Mortgage Calculator?
Before you start house hunting, it is essential to determine your budget. A mortgage calculator allows you to experiment with different scenarios—such as changing your down payment or securing a lower interest rate—to see how they impact your monthly bottom line. This empowers you to make financially sound decisions and avoid becoming "house poor."
function calculateMortgage() {
// Get input values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTerm = parseFloat(document.getElementById('loanTerm').value);
var propertyTaxYearly = parseFloat(document.getElementById('propertyTax').value);
var homeInsuranceYearly = parseFloat(document.getElementById('homeInsurance').value);
var hoaFeesMonthly = parseFloat(document.getElementById('hoaFees').value);
var errorMsg = document.getElementById('errorMsg');
var resultBox = document.getElementById('resultBox');
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm) ||
homePrice <= 0 || loanTerm <= 0) {
errorMsg.style.display = 'block';
resultBox.style.display = 'none';
return;
}
errorMsg.style.display = 'none';
// Core Mortgage Calculation Variables
var principal = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Handle edge case where interest rate is 0
var monthlyPrincipalInterest = 0;
if (interestRate === 0) {
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
// Standard Amortization Formula: M = P[r(1+r)^n]/[(1+r)^n-1]
monthlyPrincipalInterest = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Additional Monthly Costs
var monthlyTax = isNaN(propertyTaxYearly) ? 0 : propertyTaxYearly / 12;
var monthlyInsurance = isNaN(homeInsuranceYearly) ? 0 : homeInsuranceYearly / 12;
var monthlyHOA = isNaN(hoaFeesMonthly) ? 0 : hoaFeesMonthly;
// Totals
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance + monthlyHOA;
var totalCostOfLoan = (monthlyPrincipalInterest * numberOfPayments);
var totalInterest = totalCostOfLoan – principal;
// Calculate Payoff Date
var today = new Date();
var payoffYear = today.getFullYear() + parseInt(loanTerm);
var payoffMonth = today.toLocaleString('default', { month: 'long' });
var payoffDateString = payoffMonth + " " + payoffYear;
// Formatting Helper
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// Display Results
document.getElementById('totalMonthlyPayment').innerText = formatter.format(totalMonthlyPayment);
document.getElementById('piPayment').innerText = formatter.format(monthlyPrincipalInterest);
document.getElementById('taxPayment').innerText = formatter.format(monthlyTax);
document.getElementById('insPayment').innerText = formatter.format(monthlyInsurance);
document.getElementById('hoaPayment').innerText = formatter.format(monthlyHOA);
document.getElementById('loanAmountResult').innerText = formatter.format(principal);
document.getElementById('totalInterestResult').innerText = formatter.format(totalInterest);
document.getElementById('payoffDate').innerText = payoffDateString;
// Show Result Box
resultBox.style.display = 'block';
}