function calculateMortgage() {
// Get inputs
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);
var pmi = parseFloat(document.getElementById("pmi").value);
var hoaFees = parseFloat(document.getElementById("hoaFees").value);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for Price, Down Payment, Rate, and Term.");
return;
}
// Defaults for optional fields if empty
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(pmi)) pmi = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// Calculations
var principal = homePrice – downPayment;
// Handle 100% down payment (no loan)
var monthlyPI = 0;
if (principal > 0) {
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
}
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
var monthlyFees = pmi + hoaFees;
var totalMonthly = monthlyPI + monthlyTax + monthlyInsurance + monthlyFees;
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
// Update DOM
document.getElementById("resPI").innerText = formatter.format(monthlyPI);
document.getElementById("resTax").innerText = formatter.format(monthlyTax);
document.getElementById("resIns").innerText = formatter.format(monthlyInsurance);
document.getElementById("resFees").innerText = formatter.format(monthlyFees);
document.getElementById("resTotal").innerText = formatter.format(totalMonthly);
document.getElementById("resLoanAmount").innerText = formatter.format(principal);
// Show results
document.getElementById("resultContainer").style.display = "block";
}
Understanding Your Mortgage PITI Calculation
Calculating your monthly mortgage payment involves more than just repaying the bank for the money you borrowed. To get a true picture of home affordability, you must use a PITI Calculator. PITI stands for Principal, Interest, Taxes, and Insurance—the four primary components of a standard mortgage payment.
1. Principal and Interest (The Core Loan)
This is the base of your payment. The Principal is the money that goes toward paying down the loan balance (initially calculated as Home Price minus Down Payment). The Interest is the cost of borrowing that money, determined by your annual interest rate and loan term (typically 15 or 30 years).
Example: On a $350,000 home with $70,000 down, your loan amount is $280,000. At a 6.5% interest rate over 30 years, your Principal and Interest payment alone is approximately $1,770.
2. Property Taxes
Real estate taxes are assessed by your local government to fund public services. These are calculated annually but are usually split into 12 installments and collected by your lender via an escrow account. Our calculator allows you to input your specific annual tax bill to see how it impacts your monthly budget.
3. Homeowner's Insurance & PMI
Lenders require Homeowner's Insurance to protect the asset against fire, theft, and damage. If you put down less than 20% of the home's value, you may also be required to pay Private Mortgage Insurance (PMI), which protects the lender if you default.
4. HOA Fees
If you are buying a condo or a home in a planned community, don't forget Homeowner Association (HOA) fees. While these are usually paid directly to the association rather than the lender, they are a critical monthly housing expense that affects your debt-to-income ratio.
How to Use This Calculator
Simply enter the home price, your planned down payment, and the current interest rate. For the most accurate results, check local property tax rates in your area (usually 1% – 2% of home value) and estimate insurance costs. The calculator will instantly break down your total monthly obligation.