Drag coefficients of various bodies. Credit: NASA.
The drag coefficient is a dimensionless number used in aerodynamics to describe the drag of a shape. The drag coefficient, Cd, is independent of the size of the object and is usually determined in a wind tunnel. It is part of the basic drag equation:
F = 0.5 × ρ × V 2 × Cd × A
where F is the drag force, ρ (rho) is the air density, V is the air velocity, and A is the cross sectional area. All of these, except Cd, are directly measurable in a wind tunnel so Cd can be thought of the "fudge factor" that accounts for all of the aerodynamic peculiarities of a shape.