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Monthly Payment Loan Calculator Form Programming Tutorial

Published : November 17, 2014   •   Last Edited : November 24, 2025   •   Author : Adam Khoury
Learn to program a Monthly Payment Loan Calculator with simple interest calculations included. The JavaScript logic can be used with or without value gathering form controls. In this tutorial we are adding the HTML form elements that will gather values needed by the script. 
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<script>

function computeLoan(){
	var amount = document.getElementById('amount').value;
	var interest_rate = document.getElementById('interest_rate').value;
	var months = document.getElementById('months').value;
	var interest = (amount * (interest_rate * .01)) / months;
	var payment = ((amount / months) + interest).toFixed(2);
	payment = payment.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
	document.getElementById('payment').innerHTML = "Monthly Payment = $"+payment;
}
</script>
</head>
<body>
<p>Loan Amount: $<input id="amount" type="number" min="1" max="1000000" onchange="computeLoan()"></p>
<p>Interest Rate: <input id="interest_rate" type="number" min="0" max="100" value="10" step=".1" onchange="computeLoan()">%</p>
<p>Months: <input id="months" type="number" min="1" max="72" value="1" step="1" onchange="computeLoan()"></p>
<h2 id="payment"></h2>
</body>
</html>
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