Understanding your estimated monthly mortgage payment is a crucial step in the home buying process. This Mortgage Payment Calculator helps you estimate your monthly housing costs by factoring in the loan principal, interest, taxes, and insurance (often referred to as PITI).
Key Components of Your Mortgage Payment
Principal: The portion of your payment that reduces the loan balance. In the early years of a mortgage, this amount is small but grows over time.
Interest: The cost of borrowing money. This is calculated based on your annual interest rate and remaining loan balance.
Escrow Costs (Taxes & Insurance): Most lenders require you to pay 1/12th of your annual property taxes and homeowners insurance into an escrow account each month.
HOA Fees: If you buy a condo or a home in a planned community, Homeowners Association fees are usually paid separately, but we include them here for a total monthly budget view.
The Mortgage Calculation Formula
While this calculator handles the math instantly, the standard formula for calculating the monthly Principal and Interest (P&I) payment is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
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)
How Interest Rates Affect Your Payment
Even a small difference in interest rates can significantly impact your monthly payment and the total interest paid over the life of the loan. For example, on a $300,000 loan, a 1% increase in interest rate can raise the monthly payment by nearly $200 and add tens of thousands of dollars to the total cost.
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 interestRateAnnual = parseFloat(document.getElementById("interestRate").value);
var propertyTaxAnnual = parseFloat(document.getElementById("propertyTax").value);
var homeInsuranceAnnual = parseFloat(document.getElementById("homeInsurance").value);
var hoaFeesMonthly = parseFloat(document.getElementById("hoaFees").value);
// 2. Validate Inputs
if (isNaN(homePrice) || homePrice <= 0) homePrice = 0;
if (isNaN(downPayment) || downPayment < 0) downPayment = 0;
if (isNaN(loanTermYears) || loanTermYears <= 0) loanTermYears = 30;
if (isNaN(interestRateAnnual) || interestRateAnnual < 0) interestRateAnnual = 0;
if (isNaN(propertyTaxAnnual) || propertyTaxAnnual < 0) propertyTaxAnnual = 0;
if (isNaN(homeInsuranceAnnual) || homeInsuranceAnnual < 0) homeInsuranceAnnual = 0;
if (isNaN(hoaFeesMonthly) || hoaFeesMonthly 0) {
if (monthlyInterestRate > 0) {
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyInterestRate, numberOfPayments);
monthlyPI = principal * ((monthlyInterestRate * x) / (x – 1));
} else {
// No interest case
monthlyPI = principal / numberOfPayments;
}
}
// 5. Calculate Other Monthly Components
var monthlyTax = propertyTaxAnnual / 12;
var monthlyInsurance = homeInsuranceAnnual / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyInsurance + hoaFeesMonthly;
// 6. Calculate Totals
var totalAmountPaid = (monthlyPI * numberOfPayments);
var totalInterest = totalAmountPaid – principal;
var totalCostOfLoan = totalAmountPaid + (monthlyTax * numberOfPayments) + (monthlyInsurance * numberOfPayments) + (hoaFeesMonthly * numberOfPayments);
// 7. Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
var payoffString = monthNames[payoffDate.getMonth()] + " " + payoffDate.getFullYear();
// 8. Display Results (Formatting as Currency)
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
document.getElementById("totalMonthlyPayment").innerHTML = formatter.format(totalMonthly);
document.getElementById("piPayment").innerHTML = formatter.format(monthlyPI);
document.getElementById("taxPayment").innerHTML = formatter.format(monthlyTax);
document.getElementById("insPayment").innerHTML = formatter.format(monthlyInsurance);
document.getElementById("hoaPayment").innerHTML = formatter.format(hoaFeesMonthly);
document.getElementById("loanAmountResult").innerHTML = formatter.format(principal);
document.getElementById("totalInterestResult").innerHTML = formatter.format(totalInterest);
document.getElementById("totalCostResult").innerHTML = formatter.format(totalCostOfLoan);
document.getElementById("payoffDateResult").innerHTML = payoffString;
// Show results section
document.getElementById("mpc-results").style.display = "block";
// Auto-scroll to results on mobile
if (window.innerWidth < 600) {
document.getElementById("mpc-results").scrollIntoView({behavior: "smooth"});
}
}