Calculate Percentage of TextBox Value using C# in ASP.Net

Hi paul,

Please refer below code

HTML

<div>
    Amount:-
    <asp:TextBox ID="TextBox1" runat="server" />
    <br />
    Percentage:-
    <asp:TextBox ID="TextBox2" runat="server" AutoPostBack="true" />
    <br />
    <asp:Button ID="btnCalculate" Text="Calculate" OnClick="btnCalculate_Click" runat="server" />
    <br />
    Result:-
    <asp:Label ID="lblTotalAmount" runat="server" />
</div>

C#

protected void btnCalculate_Click(object sender, EventArgs e)
{
    int value = Convert.ToInt32(TextBox1.Text);
    double percent = Convert.ToDouble(TextBox2.Text);
    lblTotalAmount.Text = ((value * percent) / 100 + value).ToString();
}

Screenshot

Hope this works for you