How to use it
- Choose a shape, or “Own drag coefficient” and type Cd.
- Enter the frontal area — the area of the shadow the object casts in the direction of motion.
- Enter the speed and pick m/s, km/h or mph.
- Keep 1.225 kg/m³ for air at sea level, or enter another density (fresh water is about 1000 kg/m³).
- Enter the mass if you want the terminal velocity.
Formulas
- Drag force F = ½ × ρ × v² × Cd × A
- Power P = F × v
- Terminal velocity v_t = √(2 × m × g ÷ (ρ × Cd × A)), with g = 9.80665 m/s²
| Shape | Cd |
|---|---|
| Streamlined body | 0.04 |
| Half-sphere, round side forward | 0.42 |
| Sphere | 0.47 |
| Cone, point forward | 0.50 |
| Cube, edge forward | 0.80 |
| Long cylinder, side-on | 0.82 |
| Cube, face forward | 1.05 |
| Short cylinder, end-on | 1.15 |
| Flat plate, face-on | 1.28 |
Worked example
A skydiver-like body: Cd = 1.0, A = 0.7 m², v = 30 m/s in air of 1.225 kg/m³.
- F = 0.5 × 1.225 × 30² × 1.0 × 0.7 = 385.88 N (86.75 lbf)
- P = 385.88 × 30 = 11,576.3 W
- With 80 kg (weight 784.53 N): v_t = √(2 × 80 × 9.80665 ÷ (1.225 × 1.0 × 0.7)) = 42.78 m/s, 154 km/h
Tips
- Area and Cd go together. Car makers often quote the product Cd·A; if you have it, enter Cd = 1 and that number as the area.
- Density changes with altitude and temperature. In the standard atmosphere air at 2000 m has about 1.007 kg/m³, 18 % less than at sea level, so drag is 18 % lower.
- Terminal velocity ignores buoyancy, which is fine in air but not for slow objects sinking in water.