In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The formula is accurate only under certain conditions: the objects must have a blunt form factor and the fluid must have a large enough Reynolds number to produce turbulence behind the object. The equation is

is the drag force, which is by definition the force component in the direction of the flow velocity,

is the mass density of the fluid,

is the flow velocity relative to the object,

is the reference area, and

is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag.