Calculating your monthly mortgage payment is a crucial step in the home-buying process. This Mortgage Payment Calculator helps you estimate your monthly financial commitment based on the home price, your down payment, the interest rate, and the loan term. By understanding these numbers, you can determine exactly how much house you can afford.
How the Mortgage Formula Works
While this calculator handles the heavy lifting instantly, the underlying math uses the standard amortization formula used by lenders across the United States. The formula determines the fixed monthly payment required to pay off the loan principal and interest over a set period.
The calculation considers:
Principal (P): The home price minus your down payment. This is the actual amount you borrow.
Interest Rate (r): Your annual interest rate divided by 12 (monthly rate).
Number of Payments (n): The total number of months in your loan term (e.g., 30 years × 12 months = 360 payments).
Factors That Impact Your Monthly Payment
Even small changes in your input variables can drastically change your long-term costs:
Down Payment: A larger down payment reduces your principal loan amount, which lowers both your monthly payment and the total interest paid over the life of the loan.
Interest Rate: This is the cost of borrowing money. A lower credit score often results in a higher interest rate. Even a 0.5% difference can save or cost you tens of thousands of dollars over 30 years.
Loan Term: A 15-year mortgage will have higher monthly payments than a 30-year mortgage, but you will pay significantly less in total interest because the money is borrowed for a shorter time.
Frequently Asked Questions
Does this calculator include property taxes and insurance?
No, this calculation covers Principal and Interest (often abbreviated as P&I). Property taxes, homeowners insurance, and potentially Private Mortgage Insurance (PMI) are usually added to this number by your lender to form your total monthly escrow payment.
What is a good interest rate?
Interest rates fluctuate daily based on the bond market and economic indicators. It is best to check with local lenders for current averages based on your credit score.
function calculateMortgage() {
// 1. Get Input Values using exact IDs
var homePriceInput = document.getElementById("homePrice").value;
var downPaymentInput = document.getElementById("downPayment").value;
var interestRateInput = document.getElementById("interestRate").value;
var loanTermInput = document.getElementById("loanTerm").value;
// 2. Validate Inputs
var price = parseFloat(homePriceInput);
var down = parseFloat(downPaymentInput);
var rate = parseFloat(interestRateInput);
var years = parseFloat(loanTermInput);
// Check if values are valid numbers
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(years)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (price <= 0 || years <= 0) {
alert("Home Price and Loan Term must be greater than zero.");
return;
}
// 3. Perform Calculations
var principal = price – down;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
var monthlyInterest = (rate / 100) / 12;
var totalPayments = years * 12;
var monthlyPayment = 0;
// Handle 0% interest edge case
if (rate === 0) {
monthlyPayment = principal / totalPayments;
} else {
// Standard Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyInterest, totalPayments);
monthlyPayment = (principal * x * monthlyInterest) / (x – 1);
}
var totalPaid = monthlyPayment * totalPayments;
var totalInterest = totalPaid – principal;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + totalPayments));
var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
var payoffString = monthNames[payoffDate.getMonth()] + " " + payoffDate.getFullYear();
// 4. Update the HTML with Results using Formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("monthlyPaymentDisplay").innerHTML = formatter.format(monthlyPayment);
document.getElementById("totalPaymentDisplay").innerHTML = formatter.format(totalPaid);
document.getElementById("totalInterestDisplay").innerHTML = formatter.format(totalInterest);
document.getElementById("payoffDateDisplay").innerHTML = payoffString;
// Show the result container
document.getElementById("calcResult").style.display = "block";
}