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 years = parseFloat(document.getElementById('loanTerm').value);
var taxYear = parseFloat(document.getElementById('propertyTax').value);
var insYear = parseFloat(document.getElementById('homeInsurance').value);
var hoaMonth = parseFloat(document.getElementById('hoaFees').value);
// 2. Validation
var errorDiv = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('resultsArea');
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(years) ||
isNaN(taxYear) || isNaN(insYear) || isNaN(hoaMonth)) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
// Logic check: Down payment cannot exceed price
if (down >= price) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
errorDiv.style.display = 'none';
// 3. Calculation Logic
var principal = price – down;
var monthlyRate = (rate / 100) / 12;
var numberOfPayments = years * 12;
// Principal and Interest (P&I) Calculation
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPI = 0;
if (rate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Monthly Tax and Insurance
var monthlyTax = taxYear / 12;
var monthlyIns = insYear / 12;
// Total Monthly Payment (PITI + HOA)
var totalMonthly = monthlyPI + monthlyTax + monthlyIns + hoaMonth;
// 4. Update Display
document.getElementById('resPI').innerText = '$' + monthlyPI.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
document.getElementById('resTax').innerText = '$' + monthlyTax.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
document.getElementById('resIns').innerText = '$' + monthlyIns.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
document.getElementById('resHOA').innerText = '$' + hoaMonth.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
document.getElementById('resTotal').innerText = '$' + totalMonthly.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
// Show Results
resultsDiv.style.display = 'block';
}
Understanding Your Mortgage Payment (PITI)
Calculating your monthly mortgage payment involves more than just repaying the money you borrowed. To get an accurate picture of your monthly housing costs, you need to understand the concept of PITI. This acronym stands for Principal, Interest, Taxes, and Insurance—the four main components that make up your total monthly payment.
1. Principal and Interest
The core of your mortgage payment goes toward the Principal (the actual loan balance) and the Interest (the cost of borrowing money from the lender). In the early years of a standard 30-year fixed mortgage, the majority of your payment goes toward interest, while a smaller portion reduces your principal balance. Over time, this ratio shifts, accelerating your equity build-up.
2. Property Taxes and Insurance
Most lenders require you to pay 1/12th of your annual property taxes and home insurance premiums into an escrow account each month.
Property Taxes: assessed by your local government based on the value of your property.
Homeowners Insurance: protects your home against damage from fire, theft, and natural disasters.
3. HOA Fees
If you are buying a condo, townhouse, or a home in a planned community, you may likely owe Homeowners Association (HOA) fees. While these are rarely paid through your lender, they are a critical part of your monthly debt-to-income ratio calculation. Our calculator includes an input for HOA fees to ensure your budget is realistic.
Pro Tip: Your interest rate significantly impacts your buying power. A 1% increase in interest rates can reduce your purchasing power by approximately 10%. Always shop around for the best rate before locking in your loan.
How the Loan Term Affects Payments
Choosing between a 15-year and a 30-year mortgage is a common dilemma for homebuyers:
30-Year Fixed: Offers lower monthly payments, making expensive homes more affordable, but you pay significantly more in interest over the life of the loan.
15-Year Fixed: Comes with higher monthly payments but lower interest rates. You build equity much faster and pay far less total interest.
Using This Calculator
To get the most accurate result from our Mortgage Payment Calculator, try to find the exact property tax history of the home you are interested in (often found on real estate listing sites) and get a quote for homeowners insurance. Inputting precise figures for taxes and insurance will prevent "payment shock" when you actually close on the home.