Calculating your monthly mortgage payment is a crucial step in the homebuying process. It helps you understand affordability and manage your finances effectively. The primary components of your monthly payment are principal and interest. This calculator provides an estimate of this core part of your payment.
The Math Behind the Mortgage Payment
The standard formula used to calculate a fixed-rate mortgage payment is as follows:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment (Principal & Interest)
P = The principal loan amount (the amount you borrow)
i = Your monthly interest rate. This is your annual interest rate divided by 12. For example, a 4.5% annual rate becomes 0.045 / 12 = 0.00375 monthly.
n = The total number of payments over the loan's lifetime. This is your loan term in years multiplied by 12. For a 30-year mortgage, n = 30 * 12 = 360.
How to Use This Calculator
Simply enter the following details into the fields above:
Loan Amount: The total sum of money you are borrowing for the home.
Annual Interest Rate: The yearly interest rate on the loan, expressed as a percentage (e.g., 4.5).
Loan Term: The total duration of the loan in years (commonly 15 or 30 years).
Click "Calculate Monthly Payment" to see your estimated principal and interest payment.
Important Considerations
This calculator estimates only the principal and interest (P&I) portion of your mortgage payment. Your actual total monthly housing expense will likely be higher and may include:
Property Taxes: Annual taxes assessed by your local government.
Homeowners Insurance: Insurance to protect against damage or loss to your property.
Private Mortgage Insurance (PMI): Often required if your down payment is less than 20%.
Homeowners Association (HOA) Fees: If applicable, for properties in certain communities.
These additional costs are often collected by your lender as part of an escrow account and included in your total monthly payment, but they are not part of the P&I calculation shown here. Always consult with a mortgage professional for a complete and accurate breakdown of your potential homeownership costs.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTerm").value);
var resultContainer = document.getElementById("result-container");
var monthlyPaymentDisplay = document.getElementById("monthlyPayment");
// Input validation
if (isNaN(loanAmount) || loanAmount <= 0 ||
isNaN(annualInterestRate) || annualInterestRate < 0 ||
isNaN(loanTermYears) || loanTermYears 0) {
var numerator = monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments);
var denominator = Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1;
monthlyPayment = loanAmount * (numerator / denominator);
} else {
// Handle the case of 0% interest rate
monthlyPayment = loanAmount / numberOfPayments;
}
// Format the result to two decimal places and add currency symbol
monthlyPaymentDisplay.textContent = "$" + monthlyPayment.toFixed(2);
resultContainer.style.display = 'block';
}