Use this Javascript
<script type="text/javascript" language="javascript">
function NumericOnly(obj)
{
if(obj.value.length>0)
{
obj.value = obj.value.replace(/[^\d]+/g, ''); // This would validate the inputs for allowing only numeric chars
}
}
</script>
& call this function in text box as
<asp:TextBox ID="TextBox1" runat="server" ToolTip="Enter Only Numeric" onkeyup="NumericOnly(this);"></asp:TextBox>