function calculateMortgage() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('homePrice').value);
var down = parseFloat(document.getElementById('downPayment').value);
var rate = parseFloat(document.getElementById('interestRate').value);
var term = parseFloat(document.getElementById('loanTerm').value);
var tax = parseFloat(document.getElementById('propertyTax').value);
var insurance = parseFloat(document.getElementById('homeInsurance').value);
var hoa = parseFloat(document.getElementById('hoaFees').value);
// 2. Validate Inputs
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(term) || isNaN(tax) || isNaN(insurance) || isNaN(hoa)) {
alert("Please ensure all fields contain valid numbers.");
return;
}
// 3. Core Calculations
var principal = price – down;
// Handle edge case where principal is 0 or negative
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to Home Price.");
return;
}
var monthlyRate = (rate / 100) / 12;
var numberOfPayments = term * 12;
var monthlyPI = 0;
// Handle edge case: 0% interest rate
if (rate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
// Standard Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPI = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
var monthlyTax = tax / 12;
var monthlyInsurance = insurance / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyInsurance + hoa;
var totalCostOfLoan = (monthlyPI * numberOfPayments);
var totalInterest = totalCostOfLoan – principal;
var totalProjectedCost = totalCostOfLoan + (monthlyTax * numberOfPayments) + (monthlyInsurance * numberOfPayments) + (hoa * numberOfPayments);
// 4. Format and Display Results
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('taxMonthly').innerHTML = formatter.format(monthlyTax);
document.getElementById('insMonthly').innerHTML = formatter.format(monthlyInsurance);
document.getElementById('hoaMonthly').innerHTML = formatter.format(hoa);
document.getElementById('totalLoanAmount').innerHTML = formatter.format(principal);
document.getElementById('totalInterest').innerHTML = formatter.format(totalInterest);
document.getElementById('totalCost').innerHTML = formatter.format(totalProjectedCost);
// Show results section
document.getElementById('results').style.display = 'block';
}
Understanding Your Mortgage Payment
Calculing your mortgage payment is the first critical step in the home buying process. This Mortgage Calculator is designed to give you a comprehensive breakdown of your monthly financial obligations, going beyond simple principal and interest to include taxes, insurance, and HOA fees (PITI). Understanding these components ensures you budget accurately for your new home.
The Components of a Mortgage Payment (PITI)
Most first-time homebuyers focus solely on the loan repayment, but your monthly bill usually consists of four main parts:
Principal: The portion of your payment that pays down the actual amount you borrowed. In the early years of a 30-year loan, this amount is small but grows over time.
Interest: The cost of borrowing money. At the start of your loan term, the majority of your payment goes toward interest.
Taxes: Property taxes charged by your local government. Lenders often collect this monthly and place it in an escrow account to pay the bill annually on your behalf.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is often escrowed and paid monthly.
How Interest Rates Affect Your Buying Power
Even a small fluctuation in interest rates can significantly impact your monthly payment and the total cost of the loan. For example, on a $300,000 loan, a 1% increase in interest rate can raise your monthly payment by hundreds of dollars and cost tens of thousands more in interest over the life of the loan. Use the calculator above to experiment with different rates to see how they impact your affordability.
Principal vs. Interest Over Time
Mortgages use an amortization schedule. This means that while your total monthly principal and interest payment remains fixed for the life of a fixed-rate loan, the distribution changes. In the first few years, you are primarily paying off interest. It is often not until halfway through the loan term that you begin paying more toward the principal than the interest. Making extra payments toward the principal can drastically reduce the total interest paid and shorten the loan term.
What about HOA Fees?
If you are buying a condo or a home in a planned community, you will likely have Homeowners Association (HOA) fees. These are paid separately from your mortgage but are a mandatory monthly cost that lenders consider when calculating your debt-to-income ratio. Our calculator includes an input for HOA fees to provide a realistic view of your total monthly housing expense.