Buying a home is one of the largest financial decisions you will make. This Mortgage Payment Calculator helps you estimate your monthly housing costs by factoring in the four critical components of a monthly mortgage payment, often referred to as PITI: Principal, Interest, Taxes, and Insurance.
How the Formula Works
While the calculator handles the complex math instantly, understanding the underlying components helps you make smarter financing decisions:
Principal: The portion of your payment that goes toward reducing the loan balance. In the early years of a 30-year mortgage, this amount is small but grows over time.
Interest: The cost of borrowing money. This is calculated based on your remaining loan balance and interest rate. Initially, interest makes up the majority of your monthly payment.
Property Taxes: Fees paid to your local government, typically based on the assessed value of your home. These are often collected by the lender in escrow and paid annually on your behalf.
Homeowners Insurance: Protection for your property against damages. Like taxes, this is usually divided into monthly installments and paid via escrow.
Factors Affecting Your Monthly Payment
Several variables can significantly impact your monthly affordability:
Down Payment: A larger down payment reduces the principal loan amount, which lowers both your monthly payment and the total interest paid over the life of the loan. Putting down at least 20% also eliminates the need for Private Mortgage Insurance (PMI).
Loan Term: A 30-year term offers lower monthly payments but results in higher total interest costs. A 15-year term has higher monthly payments but saves significantly on interest.
Interest Rate: Even a fraction of a percentage point difference can change your monthly payment by hundreds of dollars and your total loan cost by tens of thousands.
Using This Data for Budgeting
Financial experts generally recommend that your total monthly housing costs (PITI) should not exceed 28% of your gross monthly income. Use the "Estimated Monthly Payment" figure generated above to see if a prospective home fits within your budget. Remember to also account for maintenance costs and utilities, which are not included in the mortgage payment.
function calculateMortgage() {
// 1. Get Input Values
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var loanTermYears = parseFloat(document.getElementById("loanTerm").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var annualPropertyTax = parseFloat(document.getElementById("propertyTax").value);
var annualHomeInsurance = parseFloat(document.getElementById("homeInsurance").value);
// 2. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(loanTermYears) || isNaN(annualInterestRate)) {
alert("Please enter valid numbers for Home Price, Down Payment, Term, and Interest Rate.");
return;
}
// Handle optional fields being empty (treat as 0)
if (isNaN(annualPropertyTax)) annualPropertyTax = 0;
if (isNaN(annualHomeInsurance)) annualHomeInsurance = 0;
// 3. Core Calculations
var loanAmount = homePrice – downPayment;
// Prevent negative loan amounts
if (loanAmount <= 0) {
alert("Down payment cannot be greater than or equal to the Home Price.");
return;
}
var monthlyInterestRate = (annualInterestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPrincipalAndInterest = 0;
if (annualInterestRate === 0) {
monthlyPrincipalAndInterest = loanAmount / numberOfPayments;
} else {
monthlyPrincipalAndInterest = loanAmount *
(monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) /
(Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
var monthlyTax = annualPropertyTax / 12;
var monthlyInsurance = annualHomeInsurance / 12;
var totalMonthlyPayment = monthlyPrincipalAndInterest + monthlyTax + monthlyInsurance;
// 4. Update UI
document.getElementById("resultsSection").style.display = "block";
// Format Currency Function
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("totalMonthlyPayment").innerText = formatter.format(totalMonthlyPayment);
document.getElementById("piPayment").innerText = formatter.format(monthlyPrincipalAndInterest);
document.getElementById("taxPayment").innerText = formatter.format(monthlyTax);
document.getElementById("insPayment").innerText = formatter.format(monthlyInsurance);
document.getElementById("loanAmountResult").innerText = formatter.format(loanAmount);
}