Write Cd = D/(A Vsquared /2) on a piece of paper. This is the formula used to measure air friction or, as it's often referred to, the drag coefficient.
Determine the variables of the bottom portion of your equation. These are determined by taking the density of the air multiplied by the velocity of the item squared, times the reference area or the area that air will be applied to. Remember that the reference area is only the area affected by the air. If your equation is for cars or vehicles, only measure the surface area of the front and other surfaces affected by the air resistance.
Work through the bottom portion of the equation so that you only have one variable on the bottom portion of the equation. The equation should read Cd = D/(your variable).
Write the final formula, Cd = D/(q*A), where q is the dynamic pressure, or r*V squared. This gives you a ratio formula showing how much air resistance is present when any given density of a fluid resistance is applied to a specific surface area.