How to calculate in grid view

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C#

<asp:GridView ID="GridView1" runat="server"
AutoGenerateColumns="false" ShowFooter="true">
<Columns>
<asp:BoundField DataField="ItemDescription" HeaderText="Item"/>
<asp:TemplateField HeaderText="Amount">
<ItemTemplate>
<asp:TextBox ID="TextBox1" runat="server"
AutoPostBack="True"
ontextchanged="TextBox1_TextChanged">
</asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

What I have tried:

string calc = "select * from calculation";
                using (SqlCommand cmd = new SqlCommand(calc, con))
                {
                    con.Open();
                    SqlDataAdapter da = new SqlDataAdapter(cmd);
                    {
                        da.Fill(dt);
                          if (dt.Rows.Count > 0)
                        {
                            GridView1.DataSource = dt;
                            GridView1.DataBind();
                        }


                        con.Close();
                    }