Drag Equation Calculator
The drag equation calculator helps you compute a force exerted on a moving object immersed in a fluid.
The formula
F = ½ρv²C_dA for drag, ½ρv²C_lA for lift
Force squares, power cubes
Drag is dynamic pressure times a coefficient times a reference area. Because dynamic pressure goes as the square of speed, the force quadruples when the speed doubles — and since power is force times speed, the power required goes up eightfold.
That cube law is why top speed is so expensive and why efficient cruising is slow. A car needing 20 hp at 60 mph needs about 160 hp at 120 mph on drag alone, and it is why the last few miles per hour of a vehicle's top speed consume a disproportionate share of its engine.
The coefficient is meaningless without knowing which area it was defined against. Automotive drag coefficients use frontal area, aerofoil coefficients use plan area, and a sphere's uses its cross-section. This is why the drag area — the product of coefficient and area — is the honest figure for comparing vehicles: a low coefficient on a large frontal area is not aerodynamic.
Both coefficients vary with Reynolds number, sometimes sharply. A sphere's drag coefficient drops abruptly at around Re = 3 × 10⁵ when its boundary layer turns turbulent, which is exactly why golf balls have dimples.