function calculateMortgage() {
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var termYears = parseFloat(document.getElementById('loanTerm').value);
var annualRate = parseFloat(document.getElementById('interestRate').value);
var yearlyTax = parseFloat(document.getElementById('propertyTax').value) || 0;
var yearlyIns = parseFloat(document.getElementById('homeInsurance').value) || 0;
var errorDisplay = document.getElementById('errorDisplay');
var resultsDiv = document.getElementById('results');
// Reset error
errorDisplay.style.display = 'none';
errorDisplay.innerText = ";
// Validation
if (isNaN(homePrice) || homePrice <= 0) {
errorDisplay.innerText = 'Please enter a valid home price.';
errorDisplay.style.display = 'block';
return;
}
if (isNaN(downPayment) || downPayment = homePrice) {
errorDisplay.innerText = 'Down payment cannot equal or exceed home price.';
errorDisplay.style.display = 'block';
return;
}
if (isNaN(annualRate) || annualRate < 0) {
errorDisplay.innerText = 'Please enter a valid interest rate.';
errorDisplay.style.display = 'block';
return;
}
// Calculations
var loanAmount = homePrice – downPayment;
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = termYears * 12;
var monthlyPI = 0;
// Handle zero interest case
if (annualRate === 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
var monthlyTax = yearlyTax / 12;
var monthlyIns = yearlyIns / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyIns;
var totalRepaid = (monthlyPI * numberOfPayments) + (monthlyTax * numberOfPayments) + (monthlyIns * numberOfPayments); // This is total out of pocket over life of loan including escrow
var totalLoanCost = monthlyPI * numberOfPayments;
var totalInterest = totalLoanCost – loanAmount;
// Formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('monthlyPaymentResult').innerText = formatter.format(totalMonthly);
document.getElementById('piResult').innerText = formatter.format(monthlyPI);
document.getElementById('taxResult').innerText = formatter.format(monthlyTax);
document.getElementById('insResult').innerText = formatter.format(monthlyIns);
// For total repaid, usually people want to know Loan Total + Interest
document.getElementById('totalRepaidResult').innerText = formatter.format(totalLoanCost);
document.getElementById('totalInterestResult').innerText = formatter.format(totalInterest);
resultsDiv.style.display = 'block';
}
Understanding Your Mortgage Calculation
Calculating your monthly mortgage payment is the first step in determining "how much house" you can afford. While the math can seem complex, our mortgage calculator simplifies the process by breaking down the essential components that make up your monthly financial obligation.
The 4 Key Components of a Mortgage Payment (PITI)
Most mortgage payments are comprised of four distinct parts, commonly referred to as PITI:
Principal: The portion of your payment that goes directly toward reducing the loan balance. In the early years of a mortgage, this amount is small, but it grows over time.
Interest: The cost of borrowing money from your lender. This usually makes up the majority of your payment at the beginning of your loan term.
Taxes: Property taxes assessed by your local government. 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 in your escrow account.
How Interest Rates Affect Your Buying Power
Even a small difference in interest rates can have a massive impact on your monthly payment and the total cost of the loan. For example, on a $300,000 loan, the difference between a 6% and a 7% interest rate is roughly $200 per month. Over the life of a 30-year loan, that 1% difference costs you over $70,000 in additional interest.
Fixed-Rate vs. Adjustable-Rate Mortgages (ARMs)
This calculator assumes a Fixed-Rate Mortgage, where your interest rate remains constant for the life of the loan. This provides stability and predictability. Conversely, an Adjustable-Rate Mortgage (ARM) has an interest rate that may change periodically based on market indices. While ARMs often start with lower rates, they carry the risk of payments increasing significantly in the future.
Frequently Asked Questions
What is Private Mortgage Insurance (PMI)?
If your down payment is less than 20% of the home's purchase price, lenders typically require you to pay Private Mortgage Insurance (PMI). This protects the lender if you default on the loan. PMI usually costs between 0.5% and 1% of the loan amount annually. Once you build 20% equity in your home, you can usually request to have PMI removed.
Does a 15-year or 30-year term make more sense?
A 15-year mortgage will have higher monthly payments because you are paying off the principal faster, but you will pay significantly less interest over the life of the loan. A 30-year mortgage offers lower monthly payments, making homes more affordable month-to-month, but results in higher total interest costs over time.
What expenses are not included in this calculator?
While this calculator covers PITI, you should also budget for:
HOA Fees: If you buy a condo or a home in a planned community, you may owe Homeowners Association dues.
Maintenance Costs: A general rule of thumb is to budget 1% of the home's value annually for repairs and maintenance.
Utilities: Water, electricity, gas, and internet costs.
How is the monthly payment calculated?
The standard formula used for fixed-rate mortgages is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
M = Total monthly payment
P = Principal loan amount
i = Monthly interest rate (Annual rate divided by 12)
n = Number of payments (Loan term in years multiplied by 12)