<script language="javascript" type="text/javascript">
function countNum()
{
var count=document.getElementById("txtText").value.length;
document.getElementById("lblCount").value=count;
}
</script>
<asp:TextBox ID="txtText" name="txtText" runat="server" Columns="80" Rows="8" TextMode="MultiLine"
Width="650px" Height="150px" οnkeydοwn="countNum();"
οnmοusemοve="countNum();" οnmοuseοut="countNum();" οnkeyup="countNum();"></asp:TextBox>