Drag equation

<math>D=C_d \times \rho \times {V^2 \over 2} \times A</math>

where

D is the force of drag,

C_d is the drag coefficient (dimensionless),

ρ is the density of the fluid,

V is the speed of the object relative to the fluid, and

A is the reference area.

Of particular importance is the V² figure, meaning that fluid drag increases with the square of velocity. A car cruising on a highway at 50mph may require only 10 horsepower to overcome air drag (and another 15 or so for other sources of drag), but that same car at 100mph requires 100hp.

The reference area A is related to, but not exactly equal to, the area of the projection of the object on a plane perpendicular to the direction of motion. It is determined by experiment.