The drag coefficient C_d is defined as

C_d = \frac{2 F_d}{\rho u^2 A}

where F_d is the drag force component in the direction of the flow velocity, ρ is the density of the fluid, u is the flow speed relative to the object and A is the reference area. Reference area is often the same as projected frontal area, but for bodies that vary in cross-sectional shape this might not be the case. For exemple, airfoils has the reference area of the nominal wing area.

The drag force can be divided into two components which are frictional drag (viscous drag) and pressure drag (drag based on shape). Some of the bodies in the table above would be described as blunt/bluff bodies, and these are the ones where pressure drag is the main component of the drag force. The shape of bluff bodies is defined as a shape where high pressure gradients in the flow field around it occurs (separation often occurs as well) – which is often leading to high drag. Friction is often a small component of drag, but when having streamlined bodies, this can still be the major contributor to drag since no separation occurs – this will be the case of low drag bodies.

One field where drag calculations are of high importance is the automotive industry, where improving aerodynamics (reducing drag) is important to reduce fuel consumption. The shape of cars have developed a lot during the years and the first car that was released to the market was shaped like a box and had therefore high pressure drag. The low speed of the first cars did not demand a streamlined shape, but nowadays the streamlined shape is used when designing cars. The drag coefficient has gone down from around 0,95 to 0,3. Drag coefficients for different car models can be found on Wikipedia where cars are listed, for example an Audi A3 has a C_d of 0,33 while a Tesla model S has a value of 0.24 [4].