How to calculate tax amount from grand total using C# and VB.Net in ASP.Net

Hi itsjayshah,

As per your query i have create the sample.

Please check the below sample if you have any doubt you can revert my reply.

HTML 

<div>
    <table>
        <tr>
            <td>
                Grand Total
            </td>
            <td>
                <asp:TextBox ID="txtGrandTotal" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Enter The Percentage
            </td>
            <td>
                <asp:TextBox ID="txtPercentage" runat="server" AutoPostBack="true" OnTextChanged="CalculateTaxAmount" />
            </td>
        </tr>
        <tr>
            <td>
                TaxAmount
            </td>
            <td>
                <asp:Label ID="lblTaxAmount" runat="server" Text="0"></asp:Label>
            </td>
        </tr>
    </table>
</div>

C#

protected void CalculateTaxAmount(object sender, EventArgs e)
{
    double totalAmount, taxAmount, grandTotal, percentage;
    grandTotal = Convert.ToDouble(txtGrandTotal.Text);
    percentage = Convert.ToDouble(txtPercentage.Text);
    totalAmount = (grandTotal * 100) / (100 + percentage);
    taxAmount = totalAmount * (percentage / 100);
    lblTaxAmount.Text = Convert.ToString(Math.Round(taxAmount, 2));
}

VB.Net

Protected Sub CalculateTaxAmount(sender As Object, e As EventArgs)
	Dim totalAmount As Double, taxAmount As Double, grandTotal As Double, percentage As Double
	grandTotal = Convert.ToDouble(txtGrandTotal.Text)
	percentage = Convert.ToDouble(txtPercentage.Text)
	totalAmount = (grandTotal * 100) / (100 + percentage)
	taxAmount = totalAmount * (percentage / 100)
	lblTaxAmount.Text = Convert.ToString(Math.Round(taxAmount, 2))
End Sub