Drag Force Calculator (Drag Equation)
Find the aerodynamic drag force on a moving object from its speed, frontal area, drag coefficient and the fluid density.
How it works
F_d = ½ · ρ · v² · C_d · A · Power P = F_d · v
The drag equation says the resistive force grows with the square of speed: doubling your speed quadruples the drag. It scales linearly with fluid density, frontal area and the shape-dependent drag coefficient. Because the power needed to overcome drag is force times speed, it grows with the cube of speed — which is exactly why fuel use and cycling effort climb so sharply at highway or sprint speeds.
Worked example
A sphere of 1 m² frontal area (C_d = 0.47) moving at 30 m/s through sea-level air (ρ = 1.225 kg/m³): F_d = ½ × 1.225 × 30² × 0.47 × 1 = ½ × 1.225 × 900 × 0.47 ≈ 259 N. The power to sustain that speed is 259 × 30 ≈ 7.8 kW.
Sources: Drag equation — Wikipedia
Frequently asked questions
Why does drag rise so fast with speed?
Drag is proportional to speed squared, and the power to overcome it is proportional to speed cubed. Going from 50 to 100 km/h roughly quadruples the drag force and needs about eight times the power.
What is the drag coefficient?
C_d is a dimensionless number capturing how streamlined a shape is. A sleek teardrop can be below 0.05, a modern car around 0.3, a sphere about 0.47 and a flat plate around 1.28.
Does this apply in water too?
Yes — the same equation works in any fluid; just use the fluid’s density. Water is about 800 times denser than air, so drag is far larger at the same speed.
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This calculator is for educational purposes. Double-check important results before acting on them.