Try this,
js function :
function isNumberKey(evt, ctrlName) {
var str = ctrlName.value;
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 && charCode != 46) || charCode > 57) {
return false;
}
else if (charCode == 46 && (str.indexOf(".") != -1)) {
return false;
}
else {
return true;
}
}
Calling way:
<asp:TextBox ID="txt_rate" runat="server" Width="103px" onkeypress="return isNumberKey(event,this)" ></asp:TextBox>