function calculateMortgage() {
// Get Inputs
var homePrice = parseFloat(document.getElementById('mcHomePrice').value);
var downPayment = parseFloat(document.getElementById('mcDownPayment').value);
var termYears = parseFloat(document.getElementById('mcLoanTerm').value);
var annualRate = parseFloat(document.getElementById('mcInterestRate').value);
var annualTax = parseFloat(document.getElementById('mcPropertyTax').value);
var annualIns = parseFloat(document.getElementById('mcInsurance').value);
// Validation
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment)) downPayment = 0;
if (isNaN(termYears) || termYears <= 0) {
alert("Please enter a valid Loan Term.");
return;
}
if (isNaN(annualRate) || annualRate < 0) {
alert("Please enter a valid Interest Rate.");
return;
}
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualIns)) annualIns = 0;
// Calculations
var loanAmount = homePrice – downPayment;
var numPayments = termYears * 12;
var monthlyRate = (annualRate / 100) / 12;
var monthlyPI = 0;
if (monthlyRate === 0) {
monthlyPI = loanAmount / numPayments;
} else {
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) / (Math.pow(1 + monthlyRate, numPayments) – 1);
}
var monthlyTax = annualTax / 12;
var monthlyIns = annualIns / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyIns;
var totalPayment = monthlyPI * numPayments;
var totalInterest = totalPayment – loanAmount;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numPayments));
var options = { year: 'numeric', month: 'long' };
var payoffString = payoffDate.toLocaleDateString('en-US', options);
// Display Results
document.getElementById('mcTotalMonthly').innerText = formatCurrency(totalMonthly);
document.getElementById('mcMonthlyPI').innerText = formatCurrency(monthlyPI);
document.getElementById('mcMonthlyTax').innerText = formatCurrency(monthlyTax);
document.getElementById('mcMonthlyIns').innerText = formatCurrency(monthlyIns);
document.getElementById('mcLoanAmount').innerText = formatCurrency(loanAmount);
document.getElementById('mcTotalInterest').innerText = formatCurrency(totalInterest);
document.getElementById('mcPayoffDate').innerText = payoffString;
// Show Results container
document.getElementById('mcResults').style.display = 'block';
}
function formatCurrency(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
Understanding Your Mortgage Calculation
Purchasing a home is one of the most significant financial decisions most people make in their lifetimes. Understanding exactly how your monthly payments are calculated is crucial for budgeting and financial planning. This mortgage calculator breaks down your payment into its core components, helping you visualize where your money goes every month.
The 4 Pillars of a Mortgage Payment (PITI)
Your monthly mortgage payment is typically composed of four main parts, commonly referred to by the acronym PITI:
Principal: This is the portion of your payment that goes directly toward paying down the loan balance (the amount you borrowed). In the early years of a mortgage, this amount is small but grows over time.
Interest: This is the cost of borrowing money, paid to the lender. Initially, interest makes up the majority of your monthly payment.
Taxes: Property taxes are assessed by your local government to fund public services like schools and roads. Lenders often collect this monthly and hold it in an escrow account to pay the bill when it's due.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is often collected monthly and paid by the lender from your escrow account.
How Interest Rates Impact Your Loan
The interest rate is arguably the most critical variable in your mortgage calculation. Even a small difference in percentage points can result in saving—or spending—tens of thousands of dollars over the life of a 30-year loan.
For example, on a $300,000 loan, the difference between a 6% and a 7% interest rate changes the monthly principal and interest payment by nearly $200. Over 30 years, that 1% difference costs an additional $70,000+ in total interest paid. This highlights the importance of shopping around for the best rate and maintaining a good credit score.
Fixed vs. Adjustable Rate Mortgages
This calculator assumes a Fixed-Rate Mortgage, where the interest rate remains the same for the entire loan term (e.g., 15 or 30 years). This provides stability, as your principal and interest payment will never change.
An Adjustable-Rate Mortgage (ARM) typically starts with a lower rate for a set period (like 5 or 7 years) but can fluctuate afterwards based on market conditions. While ARMs can offer lower initial payments, they carry the risk of payments increasing significantly in the future.
Private Mortgage Insurance (PMI)
If your down payment is less than 20% of the home's purchase price, lenders usually require Private Mortgage Insurance (PMI). This protects the lender if you stop making payments. PMI typically costs between 0.5% to 1% of the entire loan amount annually. While this calculator includes estimates for taxes and insurance, be sure to factor in PMI costs if you are making a small down payment.
Tips for Lowering Your Monthly Payment
Increase Your Down Payment: Putting more money down reduces the loan amount, which lowers your monthly principal and interest. If you hit 20%, you also avoid PMI.
Extend the Loan Term: A 30-year term will have lower monthly payments than a 15-year term, although you will pay significantly more interest over the life of the loan.
Buy Points: You can sometimes pay an upfront fee ("points") to lower your interest rate for the duration of the loan.
Shop for Insurance: Homeowners insurance rates vary. Shopping around can sometimes save you hundreds of dollars a year, reducing your monthly escrow payment.
Using This Calculator for Refinancing
This tool is also excellent for analyzing potential refinance options. To do this, simply enter your remaining loan balance as the "Home Price" (set Down Payment to 0), and input the new interest rate and term you are considering. Compare the resulting monthly payment to your current statement to see if refinancing makes financial sense.