function calculateMortgage() {
// Get inputs by ID matches exactly
var homePriceInput = document.getElementById("mc_homePrice");
var downPaymentInput = document.getElementById("mc_downPayment");
var interestRateInput = document.getElementById("mc_interestRate");
var loanTermInput = document.getElementById("mc_loanTerm");
// Parse values
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var annualRate = parseFloat(interestRateInput.value);
var years = parseFloat(loanTermInput.value);
// Validation logic
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(annualRate) || isNaN(years) || homePrice <= 0 || years = home price
if (principal <= 0) {
document.getElementById("mc_results").style.display = "block";
document.getElementById("mc_monthlyPayment").innerText = "$0.00";
document.getElementById("mc_loanAmount").innerText = "$0.00";
document.getElementById("mc_totalInterest").innerText = "$0.00";
document.getElementById("mc_totalCost").innerText = "$" + homePrice.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPayment = 0;
if (annualRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPayment = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2,
maximumFractionDigits: 2
});
// Update DOM
document.getElementById("mc_results").style.display = "block";
document.getElementById("mc_monthlyPayment").innerText = formatter.format(monthlyPayment);
document.getElementById("mc_loanAmount").innerText = formatter.format(principal);
document.getElementById("mc_totalInterest").innerText = formatter.format(totalInterest);
document.getElementById("mc_totalCost").innerText = formatter.format(totalCost);
}
How to Use This Mortgage Calculator
Purchasing a home is one of the most significant financial decisions you will make in your lifetime. Understanding the long-term costs associated with a mortgage is crucial for financial planning. Our Mortgage Calculator is designed to provide you with a clear, instant estimate of your monthly principal and interest payments based on your home price, down payment, interest rate, and loan term.
Understanding the Inputs
Home Price: The total purchase price of the real estate property you intend to buy.
Down Payment: The amount of money you pay upfront. A higher down payment reduces the principal loan amount and often secures a better interest rate.
Interest Rate: The annual percentage rate (APR) charged by the lender. Even a small difference of 0.5% can significantly impact your total repayment amount over 30 years.
Loan Term: The duration of the loan, typically 15 or 30 years. Shorter terms usually have higher monthly payments but lower total interest costs.
What Determines Your Monthly Payment?
Your monthly mortgage payment is primarily composed of Principal and Interest (often abbreviated as P&I). The principal is the money that goes toward paying off the loan balance, while interest is the cost of borrowing that money.
Please note that this calculator estimates P&I only. Most homeowners also pay for property taxes, homeowners insurance, and potentially Private Mortgage Insurance (PMI) if the down payment is less than 20%. These additional costs are usually bundled into your monthly escrow payment.
How Amortization Works
Mortgages use an amortization schedule to pay off the loan. In the early years of your loan term, a large portion of your monthly payment goes toward interest. As you continue to make payments, the principal balance decreases, meaning less interest accrues, and more of your payment goes toward the principal. Using this calculator allows you to see the total interest you will pay over the life of the loan, helping you decide if a shorter term or larger down payment is right for you.
Why Calculate Your Mortgage Before Buying?
Calculating your potential payments before viewing homes helps you set a realistic budget. It prevents "house poor" scenarios where too much of your income is tied up in housing costs, leaving little for savings, emergencies, or lifestyle expenses. Financial experts generally recommend that your total housing costs should not exceed 28% of your gross monthly income.