Calculate aerodynamic drag force and the power needed to overcome it from fluid density, speed, drag coefficient and frontal area.
Drag grows with the square of speed: doubling speed quadruples drag and multiplies the power needed by eight. Density is 1.225 kg/m³ for air at sea level and about 997 kg/m³ for water.
Drag force
How to use this tool
Type density in kg/m³ — 1.225 for air at sea level, about 997 for water.
Type the speed relative to the fluid in metres per second.
Type the drag coefficient Cd for the shape and the frontal area in m².
Force is what must be overcome; power is force times speed, the rate of energy lost to drag.
Formula & worked example
At 30 m/s in air with Cd = 0.3 and area 2 m², drag is about 330.75 N and the power to overcome it is about 9.92 kW.
Limitations — read this before you rely on it
This is the quadratic drag regime for bluff bodies at high Reynolds number. At very low speeds or for tiny particles, linear (Stokes) drag applies instead, and Cd itself varies with Reynolds number.
Frequently asked questions
F = ½ρv²CdA: drag grows with density, the square of speed, drag coefficient and frontal area.
About 0.3 for a modern car, 0.5 for a sphere, 1.0–1.3 for a flat plate facing the flow.
Because force grows with v² and power is force times speed, so doubling speed needs eight times the power.
The projected area facing the flow — for a car, roughly width times height.
At low Reynolds numbers (creeping flow), where drag is linear in speed, and near the speed of sound, where compressibility changes Cd.