Drag coefficient

Definition

Let a body move with constant speed $U > 0$ relative to a fluid of density $\rho > 0$ . Let $A > 0$ be a chosen reference area of the body, such as its frontal area. Let $F_D$ be the component, along the direction of the relative flow, of the force that the fluid exerts on the body. The drag coefficient of the body is $C_D = \frac{2 F_D}{\rho U^2 A}.$ The quantity $\frac12 \rho U^2$ is the dynamic pressure.

Remarks

Example: for a sphere of radius $a$ at small Reynolds number, Stokes' law gives $F_D = 6 \pi \mu a U$ . With $A = \pi a^2$ this gives $C_D = \frac{12 \mu}{\rho U a} = \frac{24}{\mathrm{Re}}$ , where $\mathrm{Re} = 2 \rho U a / \mu$ .

Non-example: the force component perpendicular to the flow does not enter $C_D$ ; divided in the same way it gives the lift coefficient $C_L$ .

The value of $C_D$ depends on the choice of $A$ , so the reference area must always be stated; for wings one usually takes the wing area.

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Reynolds number concept
Drag coefficient definition

Tag	`ENXZK7ZS8Z`
URI	`m2://ENXZK7ZS8Z`
Kind	definition
Writing	W0 stub
MSC	76D05, 76G25
Defines	Drag coefficient
Uses	Reynolds number
Changed	3 November 2026

Drag coefficient

Definition

Remarks

Reading path

Further reading

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