getting dropdown values for the calculation

 here DDLDiameter.Text is  a dropdown, where i need to get the text value irrespective of text ID, how to do this??

 

double velocity1 = Convert.ToDouble(AirFR3.Text) / (((0.25 * 3.14159 * (Convert.ToDouble(DDLDiameter.Text) - 10) * (Convert.ToDouble(DDLDiameter.Text) - 10) / 1000000) / 2));

Svelocity.Text = Math.Round(velocity1, 3).ToString();