Understanding your monthly mortgage payment is crucial for budgeting and financial planning. This calculator will help you estimate your Principal and Interest (P&I) payment, which is a core component of your total housing expense. Your total monthly housing expense will also include property taxes, homeowners insurance, and potentially Private Mortgage Insurance (PMI) or Homeowners Association (HOA) fees, which are not included in this calculation.
What goes into your monthly mortgage payment?
The primary components calculated here are:
Principal: The amount of money you borrow to buy your home.
Interest: The cost of borrowing money, determined by your interest rate.
The formula used is the standard mortgage payment formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment (Principal & Interest)
P = The principal loan amount
i = Your monthly interest rate (annual rate divided by 12)
n = The total number of payments over the loan's lifetime (loan term in years multiplied by 12)
By inputting the key details of your potential loan, you can get a clear estimate of your P&I payment.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTermYears").value);
var resultDiv = document.getElementById("result");
if (isNaN(loanAmount) || isNaN(annualInterestRate) || isNaN(loanTermYears) || loanAmount <= 0 || annualInterestRate < 0 || loanTermYears <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
// Check for edge case where monthlyInterestRate is 0 to avoid division by zero
var monthlyPayment;
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
resultDiv.innerHTML = "Estimated Monthly Payment (Principal & Interest): $" + monthlyPayment.toFixed(2);
}