function calculateMortgage() {
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 hoaFees = parseFloat(document.getElementById('hoaFees').value);
// Basic validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for Home Price, Down Payment, Rate, and Term.");
return;
}
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(hoaFees)) hoaFees = 0;
var loanAmount = homePrice – downPayment;
// Prevent negative loan amount
if (loanAmount <= 0) {
alert("Down payment cannot exceed or equal home price.");
return;
}
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPI = 0;
// Standard Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRate === 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
var monthlyTax = propertyTax / 12;
var monthlyIns = homeInsurance / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyIns + hoaFees;
var totalCostOfLoan = (monthlyPI * numberOfPayments);
var totalInterest = totalCostOfLoan – loanAmount;
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('displayPI').innerText = formatter.format(monthlyPI);
document.getElementById('displayTax').innerText = formatter.format(monthlyTax);
document.getElementById('displayIns').innerText = formatter.format(monthlyIns);
document.getElementById('displayHOA').innerText = formatter.format(hoaFees);
document.getElementById('displayTotal').innerText = formatter.format(totalMonthly);
document.getElementById('displayLoanAmount').innerText = formatter.format(loanAmount);
document.getElementById('displayTotalInterest').innerText = formatter.format(totalInterest);
// Show results
document.getElementById('resultsArea').style.display = 'block';
}
Understanding Your Mortgage Payment Calculation
Purchasing a home is likely the largest financial commitment you will make in your lifetime. While the listing price of a home gives you a baseline, the actual monthly cost of homeownership involves several distinct components. Our Mortgage Calculator helps you visualize the full picture known as PITI: Principal, Interest, Taxes, and Insurance.
Breaking Down PITI
Principal: The portion of your payment that pays down the actual loan balance (the money you borrowed). In the early years of a mortgage, this amount is typically smaller than the interest portion.
Interest: The cost of borrowing money. This is determined by your annual percentage rate (APR). Higher rates significantly increase your monthly payment and the total cost of the loan over time.
Taxes: Property taxes are assessed by your local government to fund schools, police, and infrastructure. These are usually paid annually but are often escrowed into your monthly mortgage payment.
Insurance: Homeowners insurance protects your property against damage. Lenders require this. Additionally, if you put down less than 20%, you may also have to pay Private Mortgage Insurance (PMI), which protects the lender.
How Down Payments Affect Your Monthly Cost
The size of your down payment plays a critical role in your mortgage math. A larger down payment reduces the principal loan amount, which lowers your monthly P&I (Principal and Interest) payment. Furthermore, reaching a 20% down payment threshold allows you to avoid PMI, which can save homeowners hundreds of dollars per month depending on the home's value.
Interest Rates vs. Loan Term
Your monthly obligation is heavily influenced by the loan term (usually 15 or 30 years) and the interest rate. A 30-year term offers lower monthly payments by spreading the debt over a longer period, but you will pay significantly more in total interest. Conversely, a 15-year term has higher monthly payments but saves you money in the long run by clearing the debt faster.
Using This Calculator for Budgeting
When planning your budget, do not look at the principal and interest alone. Always factor in HOA fees (if buying a condo or in a planned community), annual tax increases, and maintenance costs. Financial experts generally recommend that your total housing costs should not exceed 28% of your gross monthly income.