In physics, the drag equation gives the drag experienced by an object moving through a fluid.
- D is the force of drag,
- Cd is the drag coefficient (dimensionless),
- ρ is the density of the fluid,
- V is the velocity of the object relative to the fluid, and
- A is the reference area.
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.