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<!DOCTYPE html> <html> <body> <h1>How to Auto Calculate Price in Javascript</h1> <form name="frm-pin" method="post" action="pin-index-a.php"> <input type="hidden" name="mode" value="PinRequest" /> <label class="w3-text-green"><b>Number Of Pin</b></label> <select name="tot_pin_requested" onchange="calculateAmount(this.value)" required> <option value="" disabled selected>Choose your option</option> <option value="1">1</option> <option value="2">2</option> <option value="3">3</option> <option value="4">4</option> <option value="5">5</option> <option value="6">6</option> <option value="7">7</option> <option value="8">8</option> <option value="9">9</option> <option value="10">10</option> </select> <label><b>Total Amount</b></label> <input class="w3-input w3-border" name="tot_amount" id="tot_amount" type="text" readonly> <script> function calculateAmount(val) { var tot_price = val * 100; /*display the result*/ var divobj = document.getElementById('tot_amount'); divobj.value = tot_price; } </script> <body> </html>