function calculateMortgage() {
// Get Input Values
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var termYears = parseInt(document.getElementById("loanTerm").value);
var annualRate = parseFloat(document.getElementById("interestRate").value);
var propertyTax = parseFloat(document.getElementById("propertyTax").value);
var homeInsurance = parseFloat(document.getElementById("homeInsurance").value);
var pmiCost = parseFloat(document.getElementById("pmiCost").value);
var hoaFees = parseFloat(document.getElementById("hoaFees").value);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(termYears) || isNaN(annualRate)) {
alert("Please enter valid numbers for Price, Down Payment, Term, and Interest Rate.");
return;
}
// Handle optional fields if empty/NaN
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(pmiCost)) pmiCost = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// Core Calculation Logic
var principal = homePrice – downPayment;
// Prevent negative principal
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = termYears * 12;
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPrincipalInterest = 0;
if (annualRate === 0) {
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
monthlyPrincipalInterest = (principal * monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Escrow and Fees
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
var monthlyEscrow = monthlyTax + monthlyInsurance;
var monthlyFees = pmiCost + hoaFees;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyEscrow + monthlyFees;
var totalCostOfLoan = (monthlyPrincipalInterest * numberOfPayments);
var totalInterestPaid = totalCostOfLoan – principal;
// Date Calculation
var today = new Date();
today.setMonth(today.getMonth() + numberOfPayments);
var payoffMonth = today.toLocaleString('default', { month: 'long' });
var payoffYear = today.getFullYear();
var payoffDateString = payoffMonth + " " + payoffYear;
// Formatting Currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// Output to DOM
document.getElementById("res-pi").innerHTML = formatter.format(monthlyPrincipalInterest);
document.getElementById("res-escrow").innerHTML = formatter.format(monthlyEscrow);
document.getElementById("res-fees").innerHTML = formatter.format(monthlyFees);
document.getElementById("res-total").innerHTML = formatter.format(totalMonthlyPayment);
document.getElementById("res-loan-amount").innerHTML = formatter.format(principal);
document.getElementById("res-total-interest").innerHTML = formatter.format(totalInterestPaid);
document.getElementById("res-payoff-date").innerHTML = payoffDateString;
// Show Result Section
document.getElementById("calc-result").style.display = "block";
}
Understanding Your Mortgage Calculation
Buying a home is likely the largest financial commitment you will make in your lifetime. Using a comprehensive Mortgage Payment Calculator is essential to ensure that your dream home fits within your budget. While many buyers focus solely on the list price of the home, the actual monthly obligation includes several other factors such as interest, taxes, and insurance.
Key Components of Your Monthly Payment
When you use the calculator above, you will notice the payment is broken down into specific categories. Understanding these helps you plan financially:
Principal & Interest (P&I): This is the core of your loan. The principal repays the amount you borrowed, while the interest is the cost of borrowing that money. Early in your loan term, most of this payment goes toward interest.
Property Taxes: Local governments assess taxes on real estate to fund schools and services. These are often collected by your lender in an escrow account and paid annually on your behalf.
Homeowners Insurance: Lenders require you to insure the property against damage. Like taxes, this is typically paid monthly into an escrow account.
PMI (Private Mortgage Insurance): If your down payment is less than 20% of the home's value, lenders usually require PMI to protect them against default. This cost vanishes once you reach 20% equity.
How Interest Rates Impact Your Loan
Even a small change in interest rates can drastically change your monthly payment and total loan cost. For example, on a $300,000 loan over 30 years:
At 6.0% interest, your monthly P&I is roughly $1,799.
At 7.0% interest, your monthly P&I jumps to $1,996.
That 1% difference costs you nearly $200 extra per month and over $70,000 in additional interest over the life of the loan. Use the calculator to test different rate scenarios to see how buying down points or improving your credit score could save you money.
Choosing the Right Loan Term
While a 30-year fixed mortgage is the most common choice due to lower monthly payments, a 15-year term can offer significant savings. 15-year loans typically come with lower interest rates and, because the repayment period is half as long, the total interest paid is often less than half of what you would pay on a 30-year loan. However, the monthly payment will be significantly higher.