get dropdownlist selected value into double variable asp.net web forms

I need to pass Drop down list value to double variable.

    <asp:DropDownList ID="Drpdownlist" runat="server" Font-Names="Arial" Font-Size="12pt" Height="20px" Width="100px">
        <asp:ListItem Selected="True" Value="0.5">0.5 %</asp:ListItem>
        <asp:ListItem Value="1">1 %</asp:ListItem>
        <asp:ListItem Value="1.5">1.5 %</asp:ListItem>
        <asp:ListItem Value="2">2 %</asp:ListItem>
        <asp:ListItem Value="2.5">2.5 %</asp:ListItem>
        <asp:ListItem Value="3">3 %</asp:ListItem>
        <asp:ListItem Value="3.5">3.5 %</asp:ListItem>
        <asp:ListItem Value="4">4 %</asp:ListItem>
        <asp:ListItem Value="4.5">4.5 %</asp:ListItem>
        <asp:ListItem Value="5">5 %</asp:ListItem>
    </asp:DropDownList>

There is variable

double rate = .5;