function calculateMortgage() {
// Get input values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTermYears = parseInt(document.getElementById('loanTerm').value);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTermYears)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (downPayment >= homePrice) {
alert("Down payment cannot be equal to or greater than the home price.");
return;
}
// Calculation Logic
var principal = homePrice – downPayment;
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
if (interestRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var pow = Math.pow(1 + monthlyInterestRate, numberOfPayments);
monthlyPayment = principal * ((monthlyInterestRate * pow) / (pow – 1));
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Display Results
document.getElementById('monthlyPaymentDisplay').innerHTML = "$" + monthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('principalDisplay').innerHTML = "$" + principal.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('totalInterestDisplay').innerHTML = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('totalCostDisplay').innerHTML = "$" + totalCost.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Show result section
document.getElementById('results').style.display = "block";
}
Understanding Your Mortgage Calculation
Using a mortgage payment calculator is an essential step in the home buying process. It helps prospective buyers understand the financial commitment involved in purchasing a property by estimating the monthly principal and interest payments.
How Mortgage Payments Are Calculated
Your monthly mortgage payment is primarily determined by four key factors:
Home Price: The total sale price of the property you intend to purchase.
Down Payment: The amount of cash you pay upfront. A higher down payment reduces the loan amount (principal) and can lower your interest rate and monthly payments.
Interest Rate: The annual percentage rate (APR) charged by the lender. Even a small difference in rates can significantly impact the total cost of the loan over 15 or 30 years.
Loan Term: The length of time you have to repay the loan. A 30-year term offers lower monthly payments but results in higher total interest paid compared to a 15-year term.
Principal vs. Interest
In the early years of a mortgage, a large portion of your monthly payment goes toward paying off the interest. As time passes and the principal balance decreases, a larger portion of the payment goes toward the principal. This process is known as amortization.
Additional Costs to Consider
While this calculator estimates your Principal and Interest (P&I), keep in mind that your actual monthly housing cost may include:
Property Taxes: Levied by local governments, usually based on the assessed value of the home.
Homeowners Insurance: Protects your home against damages and liability.
PMI (Private Mortgage Insurance): Required by lenders if your down payment is less than 20% of the home price.
HOA Fees: Monthly or annual fees if the property is part of a Homeowners Association.
Use our calculator above to experiment with different home prices, down payments, and interest rates to find a mortgage plan that fits your budget comfortably.