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 loanTerm = parseFloat(document.getElementById("loanTerm").value);
var propertyTax = parseFloat(document.getElementById("propertyTax").value);
var homeInsurance = parseFloat(document.getElementById("homeInsurance").value);
// Validation to prevent errors
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for Home Price, Down Payment, Interest Rate, and Loan Term.");
return;
}
// Handle optional fields being empty
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
// Derived variables
var principal = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Calculate Monthly Principal & Interest (P&I)
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPI = 0;
if (interestRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Calculate Monthly Tax and Insurance
var monthlyTax = propertyTax / 12;
var monthlyIns = homeInsurance / 12;
// Total Monthly Payment
var totalMonthly = monthlyPI + monthlyTax + monthlyIns;
// Total Cost Calculations
var totalPaymentsAmount = monthlyPI * numberOfPayments;
var totalInterest = totalPaymentsAmount – principal;
// Display Results
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("totalMonthlyDisplay").innerHTML = formatter.format(totalMonthly);
document.getElementById("piDisplay").innerHTML = formatter.format(monthlyPI);
document.getElementById("taxDisplay").innerHTML = formatter.format(monthlyTax);
document.getElementById("insDisplay").innerHTML = formatter.format(monthlyIns);
document.getElementById("loanAmountDisplay").innerHTML = formatter.format(principal);
document.getElementById("totalInterestDisplay").innerHTML = formatter.format(totalInterest);
// Show result section
document.getElementById("resultSection").style.display = "block";
}
Understanding Your Mortgage Calculation
Calculating your monthly mortgage payment is a critical step in the home-buying process. This Mortgage Payment Calculator helps you estimate not just the repayment of your loan, but the total "PITI" (Principal, Interest, Taxes, and Insurance) that constitutes your actual monthly housing obligation.
Key Factors Affecting Your Payment
Principal: The amount of money you borrow to buy the house. This is the home price minus your down payment. A larger down payment reduces your principal and your monthly payment.
Interest Rate: The cost of borrowing money, expressed as a percentage. Even a small difference in rates (e.g., 6.0% vs 6.5%) can add up to tens of thousands of dollars over the life of a 30-year loan.
Loan Term: The duration of the loan. A 30-year term offers lower monthly payments but results in higher total interest costs compared to a 15-year term.
The Mortgage Formula Explained
Mortgage amortization uses a specific formula to ensure your payments are equal every month while gradually shifting from paying off interest to paying off the principal. The mathematical formula used in this calculator is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
M = Total monthly payment
P = Principal loan amount
i = Monthly interest rate (Annual rate divided by 12)
n = Total number of months (Loan term in years multiplied by 12)
Why Include Taxes and Insurance?
Many first-time homebuyers focus solely on the principal and interest. However, most lenders require an escrow account where they collect 1/12th of your annual property taxes and homeowners insurance along with your mortgage payment. This ensures these bills are paid on time. As shown in the calculator results above, these additional costs can significantly increase your monthly budget requirements.