A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial. This calculator helps you estimate your Principal and Interest (P&I) payment, which is a core component of your total housing expense.
The calculation is based on the amortization formula, which determines the fixed periodic payment required to fully pay off a loan over a specific period. The formula is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment
P = The principal loan amount (the amount you borrow)
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)
Loan Amount: This is the total sum you are borrowing from the lender to purchase your property.
Annual Interest Rate: This is the yearly percentage charged by the lender on the outstanding loan balance. A lower interest rate means a lower monthly payment and less interest paid over the life of the loan.
Loan Term (Years): This is the duration over which you agree to repay the loan. Common terms include 15, 20, and 30 years. A shorter term typically results in higher monthly payments but less total interest paid. A longer term means lower monthly payments but more interest paid overall.
Important Note: This calculator provides an estimate for the Principal and Interest portion of your mortgage payment only. Your actual monthly housing expense will likely be higher as it often includes property taxes, homeowner's insurance (these are often escrowed by the lender), and potentially Private Mortgage Insurance (PMI) or Homeowner Association (HOA) fees.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTermYears").value);
var resultDisplay = document.getElementById("mortgage-result").querySelector("span");
// Input validation
if (isNaN(loanAmount) || loanAmount <= 0 ||
isNaN(annualInterestRate) || annualInterestRate <= 0 ||
isNaN(loanTermYears) || loanTermYears <= 0) {
resultDisplay.textContent = "Please enter valid positive numbers for all fields.";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
// Check for potential calculation errors (e.g., extremely high rates leading to overflow)
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultDisplay.textContent = "Calculation error. Please check your inputs.";
return;
}
resultDisplay.textContent = "$" + monthlyPayment.toFixed(2);
}