*Estimates only. Actual payments may vary by lender.
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 = parseInt(document.getElementById('loanTerm').value);
var propertyTax = parseFloat(document.getElementById('propertyTax').value);
var homeInsurance = parseFloat(document.getElementById('homeInsurance').value);
// Validation
if (isNaN(homePrice) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for Home Price, Interest Rate, and Loan Term.");
return;
}
// Handle optional fields for cleaner logic
if (isNaN(downPayment)) downPayment = 0;
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
// Core Calculation Logic
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Calculate 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 * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
// Calculate Taxes and Insurance
var monthlyTax = propertyTax / 12;
var monthlyIns = homeInsurance / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyIns;
// Formatting Output (USD)
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// Display Results
document.getElementById('displayPI').innerHTML = formatter.format(monthlyPI);
document.getElementById('displayTax').innerHTML = formatter.format(monthlyTax);
document.getElementById('displayIns').innerHTML = formatter.format(monthlyIns);
document.getElementById('displayTotal').innerHTML = formatter.format(totalMonthlyPayment);
// Show the result area
document.getElementById('results-area').style.display = 'block';
}
Understanding Your Mortgage Calculation
Purchasing a home is one of the largest financial decisions most people make. Using a Mortgage Payment Calculator is essential to understand your monthly obligations before signing on the dotted line. This tool breaks down your monthly payment into its core components: Principal, Interest, Taxes, and Insurance (often referred to as PITI).
How the Mortgage Formula Works
While the calculator does the heavy lifting, it helps to understand the math behind the numbers. The standard formula used to calculate your monthly principal and interest payment is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
M = Total monthly payment
P = Principal loan amount (Home Price minus Down Payment)
i = Monthly interest rate (Annual rate divided by 12)
n = Total number of payments (Loan term in years multiplied by 12)
Key Factors Affecting Your Payment
Several variables can significantly impact your monthly mortgage costs:
Down Payment: A larger down payment reduces your principal loan amount. If you put down less than 20%, you may also be required to pay Private Mortgage Insurance (PMI), which increases your monthly costs.
Interest Rate: Even a small difference in your interest rate can save or cost you thousands of dollars over the life of the loan. Rates fluctuate based on the economy and your personal credit score.
Loan Term: A 30-year term offers lower monthly payments but results in more interest paid over time. A 15-year term has higher monthly payments but saves significantly on total interest costs.
Property Taxes & Insurance: These are often bundled into your monthly payment via an escrow account. They vary widely by location and property value.
Example Scenario
Let's look at a realistic example. Suppose you purchase a home for $350,000 with a 20% down payment ($70,000). Your loan amount is $280,000. If you secure a 30-year fixed-rate mortgage at 6.5%:
Principal & Interest: ~$1,770 per month
Property Tax (est. 1.2%): ~$350 per month
Home Insurance (est.): ~$100 per month
Total Estimated Payment: ~$2,220 per month
Use the calculator above to adjust these numbers based on your specific loan offer and local tax rates to see exactly what you can afford.