Understanding exactly how much home you can afford is the first step in the home buying journey. This Mortgage Payment Calculator helps you estimate your monthly housing costs by factoring in the principal, interest, taxes, and insurance (often referred to as PITI).
The Formula Behind the Calculation
While our calculator handles the heavy lifting instantly, understanding the math can help you make better financial decisions. The core mortgage payment formula calculates the monthly principal and interest payment:
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)
Key Factors Affecting Your Payment
Down Payment: putting more money down upfront reduces your principal loan amount. A down payment of 20% or more typically helps you avoid Private Mortgage Insurance (PMI).
Interest Rate: Even a fraction of a percentage point can significantly change your monthly payment and the total interest paid over the life of the loan. Rates are influenced by your credit score and current market conditions.
Property Taxes & Insurance: These are often held in an escrow account and paid by your lender on your behalf. They are added to your monthly principal and interest payment. Our calculator allows you to input specific annual amounts for these costs to give you a true "out-of-pocket" monthly estimate.
30-Year vs. 15-Year Mortgages
Choosing your loan term is a trade-off. A 30-year term offers lower monthly payments, making the home more affordable month-to-month, but you will pay significantly more in interest over the life of the loan. A 15-year term has higher monthly payments, but you build equity faster and save thousands in interest costs.
function calculateMortgage() {
// 1. Get Input Values
var homePrice = parseFloat(document.getElementById("mpc-home-price").value);
var downPayment = parseFloat(document.getElementById("mpc-down-payment").value);
var interestRate = parseFloat(document.getElementById("mpc-interest-rate").value);
var years = parseInt(document.getElementById("mpc-loan-term").value);
var annualTax = parseFloat(document.getElementById("mpc-property-tax").value);
var annualInsurance = parseFloat(document.getElementById("mpc-insurance").value);
var monthlyHOA = parseFloat(document.getElementById("mpc-hoa").value);
// 2. Validate Inputs
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0;
}
if (isNaN(interestRate) || interestRate < 0) {
interestRate = 0;
}
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
if (isNaN(monthlyHOA)) monthlyHOA = 0;
// 3. Calculation Logic
var principal = homePrice – downPayment;
// Prevent negative principal
if (principal < 0) {
alert("Down payment cannot be greater than home price.");
return;
}
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPI = 0;
// Handle 0% interest case
if (interestRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
// Standard Mortgage Formula
var mathPow = Math.pow(1 + monthlyInterestRate, numberOfPayments);
monthlyPI = principal * ((monthlyInterestRate * mathPow) / (mathPow – 1));
}
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance + monthlyHOA;
var totalInterest = (monthlyPI * numberOfPayments) – principal;
// 4. Update UI
// Helper function for formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("mpc-total-monthly").innerText = formatter.format(totalMonthlyPayment);
document.getElementById("mpc-pi-monthly").innerText = formatter.format(monthlyPI);
document.getElementById("mpc-tax-monthly").innerText = formatter.format(monthlyTax);
document.getElementById("mpc-ins-monthly").innerText = formatter.format(monthlyInsurance);
document.getElementById("mpc-hoa-display").innerText = formatter.format(monthlyHOA);
document.getElementById("mpc-loan-amount").innerText = formatter.format(principal);
document.getElementById("mpc-total-interest").innerText = formatter.format(totalInterest);
// Show results section
document.getElementById("mpc-results").style.display = "block";
}