Hi RichardSa,
Instead of visible false use style properties and set display attribute to none.
use innerHTML instead of value in javascript function.
Refer below modified code.
HTML
<div class="row">
<div class="col-sm-5">
<div class="form-group">
<div class="input-group">
<asp:TextBox ID="TextBox1" runat="server" CssClass="form-control" Text="" Font-Size="14pt" placeholder="10" onKeyup="calculate();" />
<asp:Label ID="txtval" runat="server" Style="display: none" Text="10"></asp:Label>
</div>
</div>
</div>
<div class="col-sm-5">
<div class="form-group">
<div class="input-group">
<asp:Label ID="LblResult" runat="server" Text="" Font-Size="14pt"></asp:Label>
</div>
</div>
</div>
</div>
<script type="text/javascript">
function calculate() {
var txt1 = document.getElementById('TextBox1').value;
var txt2 = document.getElementById('txtval').innerHTML;
var myResult = parseFloat(txt1) * parseFloat(txt2);
document.getElementById('LblResult').innerHTML = myResult;
}
</script>
Screenshot