Calculate percentage of TextBox value using JavaScript in ASP.Net

I wanted to calculate Textbox value.Suppose i have amount RS.1000. So if i wanted to calculate additional 5% or 6% of that amount,it should generate automatically on textbox when i will click inside the textbox.so i have the following javascript code.

<script type="text/javascript">

        function calculate(amount) {
            var bankcharge = document.getElementById("<%=txtBankCharge.ClientID%>").value;
            var vat = document.getElementById("<%=txtvatt.ClientID%>").value;
            var discount = document.getElementById("<%=txtDiscount.ClientID%>").value;
                      
            var sum = 0;
           
            $(".amt").each(function () {
                
                if  (isNaN(this.value))
                {
                    alert("Please enter numbers only");
                    return false;
                }
               
                if (!isNaN(this.value) && this.value.length != 0) {
                    sum += parseFloat(this.value);

                }
                
            });
                
           
            if (bankcharge.length > 0) {
                if (isNaN(bankcharge)) {
                    alert("Bank charge should be numbers only !!");
                    return false;
                }
                else {
                    sum = sum + parseFloat(bankcharge);
                }
            }

            if (vat.length > 0)
            {
                if (isNaN(vat)) {
                    alert("VAT should be numbers only !!");
                    return false;
                }
                else {
                    sum = sum + parseFloat(vat);
                }
            }


            if (discount.length > 0) {
                if (isNaN(discount)) {
                    alert("Discount amount should be numbers only !!");
                    return false;
                }
                else {
                    sum = sum - parseFloat(discount);
                }
            }
            var atemp = sum.toString().split(".");
            if (atemp.length > 1) {
                sum = sum.toFixed(2);
            }
          
            document.getElementById("<%=txtTotal.ClientID%>").value = sum;
            document.getElementById("<%=txtAmountInWords.ClientID%>").value = convertNumberToWords(sum);

        }
</script>