Hi ! First at all thanks for read, I'm new in the forum ... So the problem is, I would calculate the drag coeff of a car but in the coeff formula I would like to know which AREA is considered for this. drag coeff is given by = 2F / ρ(V^2) A Where : F is the drag force ρ is the mass density of the fluid (air in this case) V is the speed of the car and A is the reference Area is this that I can't understand. In Wikipedia (I know is not a good source) they say that this area is the projected area of the shape (vehicule) but if so... a sphere and a half sphere should have the same drag coeff. Or is not the case. It makes me doubt about this "orthogonal surface" calculation, So this area is for exemple for a cube, side by side ? or for a sphere 4∏r^2 ? and for a half sphere 2∏r^2 ??