A drag count ${\displaystyle \Delta C_{\mathrm {d} }\,}$  is defined as:

${\displaystyle \Delta C_{\mathrm {d} }=10^{4}{\dfrac {2F_{\mathrm {d} }}{\rho v^{2}A}}\,,}$  ^[3]

where:

${\displaystyle F_{\mathrm {d} }\,}$  is the drag force, which is by definition the force component in the direction of the flow velocity,^[a]

${\displaystyle \rho \,}$  is the mass density of the fluid,^[b]

${\displaystyle v\,}$  is the speed of the object relative to the fluid, and

${\displaystyle A\,}$  is the reference area.

The drag coefficient is used to compare the solutions of different geometries by means of a dimensionless number. A drag count is more user-friendly than the drag coefficient, as the latter is usually much less than 1. A drag count of 200 to 400 is typical for an airplane at cruise.^[4] A reduction of one drag count on a subsonic civil transport airplane means about 200 lb (91 kg) more in payload.^[5]

• Drag coefficient
• Zero-lift drag coefficient