function calculateMortgage() {
// 1. 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 = parseFloat(document.getElementById("loanTerm").value);
var propertyTaxYearly = parseFloat(document.getElementById("propertyTax").value);
var homeInsuranceYearly = parseFloat(document.getElementById("homeInsurance").value);
var hoaFeesMonthly = parseFloat(document.getElementById("hoaFees").value);
// 2. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for the core fields (Price, Down Payment, Rate, Term).");
return;
}
// Handle optional fields being empty
if (isNaN(propertyTaxYearly)) propertyTaxYearly = 0;
if (isNaN(homeInsuranceYearly)) homeInsuranceYearly = 0;
if (isNaN(hoaFeesMonthly)) hoaFeesMonthly = 0;
// 3. Calculation Logic
var principal = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPrincipalInterest = 0;
// Handle 0% interest edge case
if (interestRate === 0) {
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
// Standard Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var mathPower = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPrincipalInterest = principal * ((monthlyRate * mathPower) / (mathPower – 1));
}
var monthlyTax = propertyTaxYearly / 12;
var monthlyInsurance = homeInsuranceYearly / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance + hoaFeesMonthly;
var totalCostOfLoan = (monthlyPrincipalInterest * numberOfPayments);
var totalInterestPaid = totalCostOfLoan – principal;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var options = { month: 'long', year: 'numeric' };
var payoffString = payoffDate.toLocaleDateString('en-US', options);
// 4. Update UI
// Helper function for currency formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("resPrincipalInterest").innerHTML = formatter.format(monthlyPrincipalInterest);
document.getElementById("resTax").innerHTML = formatter.format(monthlyTax);
document.getElementById("resInsurance").innerHTML = formatter.format(monthlyInsurance);
document.getElementById("resHOA").innerHTML = formatter.format(hoaFeesMonthly);
document.getElementById("resTotal").innerHTML = formatter.format(totalMonthlyPayment);
document.getElementById("resTotalInterest").innerHTML = formatter.format(totalInterestPaid);
document.getElementById("resPayoffDate").innerHTML = payoffString;
// Show results container
document.getElementById("resultsArea").style.display = "block";
}
How to Calculate Your Mortgage Payments
Understanding your monthly mortgage payment is the first step toward homeownership. This Mortgage Payment Calculator breaks down the costs associated with your loan, giving you a clear picture of what you will owe every month. Unlike simple calculators that only look at principal and interest, this tool factors in property taxes, homeowners insurance, and HOA fees for a realistic estimate.
The Components of a Mortgage Payment
Your monthly check to the bank is typically made up of four specific parts, often referred to by the acronym PITI:
Principal: The portion of your payment that pays down the outstanding balance of your loan. In the beginning, this amount is small, but it grows over time as you pay down the debt.
Interest: The cost of borrowing money. This is calculated based on your annual percentage rate (APR). In the early years of a mortgage, the majority of your payment goes toward interest.
Taxes: Property taxes are usually collected by your lender and held in an escrow account to be paid to your local government annually. This calculator divides your annual tax bill by 12 to find the monthly impact.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is often paid via escrow.
How the Formula Works
While the taxes and insurance are simple additions, calculating the Principal and Interest (P&I) requires a standard amortization formula:
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 = Number of payments (Loan term in years multiplied by 12)
Strategies to Lower Your Monthly Payment
If the result from the mortgage calculator is higher than your budget allows, consider these strategies:
Increase your Down Payment: Putting more money down reduces the Principal (P), which lowers both your monthly payment and the total interest paid over the life of the loan.
Extend the Loan Term: Switching from a 15-year to a 30-year term will drastically lower your monthly obligation, though you will pay significantly more in interest over time.
Shop for Lower Insurance: Homeowners insurance rates vary. Shopping around can save you $20-$50 per month.
Avoid HOA Fees: Condos and planned communities often carry Homeowners Association fees. Buying a home without an HOA can save you hundreds per month.
Use the calculator above to experiment with these numbers and find a mortgage scenario that fits your financial goals.