Please enter valid positive numbers for all fields.
Estimated Monthly Payment
$0.00
$0.00
Principal & Interest
$0.00
Property Tax
$0.00
Home Insurance
Total Loan Amount: $0
Total Interest Paid: $0
Understanding Your Mortgage Calculation
Buying a home is one of the largest financial decisions you will make. Using a mortgage calculator helps you understand exactly how much you can afford by breaking down the monthly costs associated with a home loan. This tool takes into account the principal loan amount, interest rate, and additional costs like property taxes and insurance.
How is the Monthly Payment Calculated?
Your monthly mortgage payment is primarily composed of four parts, often referred to as PITI:
Principal: The money you borrowed to buy the house.
Interest: The cost of borrowing that money.
Taxes: Property taxes paid to your local government, usually collected in escrow.
Insurance: Homeowners insurance to protect the property.
The Amortization Formula
The core principal and interest payment is calculated using the standard amortization formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where M is the total monthly payment, P is the principal loan amount, i is the monthly interest rate, and n is the number of payments over the loan's lifetime.
Factors Affecting Your Mortgage
Down Payment: putting more money down upfront reduces the principal loan amount, which lowers your monthly payments and the total interest paid over the life of the loan.
Loan Term: A 30-year term typically offers lower monthly payments compared to a 15-year term, but you will pay significantly more in interest over time.
Interest Rate: Even a small difference in percentage points can equal thousands of dollars in savings or costs over the life of a mortgage. Your credit score significantly impacts the rate lenders offer you.
Why Include Taxes and Insurance?
Many simple calculators only show Principal and Interest. However, most lenders require you to pay property taxes and homeowners insurance into an escrow account monthly. This calculator includes these estimates to give you a more realistic view of your actual monthly housing expense.
function calculateMortgage() {
// Get input values
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var loanTermYears = parseFloat(document.getElementById("loanTerm").value);
var interestRateAnnual = parseFloat(document.getElementById("interestRate").value);
var propertyTaxAnnual = parseFloat(document.getElementById("propertyTax").value);
var homeInsuranceAnnual = parseFloat(document.getElementById("homeInsurance").value);
// Error handling elements
var errorDiv = document.getElementById("errorMessage");
var resultsBox = document.getElementById("resultsBox");
// Validate inputs
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(loanTermYears) || isNaN(interestRateAnnual) ||
homePrice <= 0 || loanTermYears <= 0) {
errorDiv.style.display = "block";
resultsBox.style.display = "none";
return;
}
// Set defaults for optional fields if empty/NaN but keep logic valid
if (isNaN(propertyTaxAnnual)) propertyTaxAnnual = 0;
if (isNaN(homeInsuranceAnnual)) homeInsuranceAnnual = 0;
errorDiv.style.display = "none";
// Calculation Logic
var principal = homePrice – downPayment;
if (principal <= 0) {
errorDiv.textContent = "Down payment cannot be greater than or equal to Home Price.";
errorDiv.style.display = "block";
resultsBox.style.display = "none";
return;
}
var monthlyInterestRate = (interestRateAnnual / 100) / 12;
var numberOfPayments = loanTermYears * 12;
// Amortization calculation
var monthlyPrincipalInterest = 0;
if (interestRateAnnual === 0) {
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
monthlyPrincipalInterest = principal *
(monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) /
(Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
var monthlyTax = propertyTaxAnnual / 12;
var monthlyInsurance = homeInsuranceAnnual / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance;
var totalInterestPaid = (monthlyPrincipalInterest * numberOfPayments) – principal;
// Formatter
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// Display Results
document.getElementById("totalMonthlyPayment").textContent = formatter.format(totalMonthlyPayment);
document.getElementById("principalInterest").textContent = formatter.format(monthlyPrincipalInterest);
document.getElementById("taxDisplay").textContent = formatter.format(monthlyTax);
document.getElementById("insuranceDisplay").textContent = formatter.format(monthlyInsurance);
document.getElementById("loanAmountResult").textContent = formatter.format(principal);
document.getElementById("totalInterestResult").textContent = formatter.format(totalInterestPaid);
resultsBox.style.display = "block";
}