Drag Coefficient Calculator Formula
Understand the math behind the drag coefficient calculator. Each variable explained with a worked example.
Formulas Used
Drag Force
drag_force = 0.5 * cd * density * area * pow(velocity, 2)Dynamic Pressure
dynamic_pressure = 0.5 * density * pow(velocity, 2)Variables
| Variable | Description | Default |
|---|---|---|
cd | Drag Coefficient (Cd) | 0.47 |
density | Fluid Density(kg/m3) | 1.225 |
area | Reference Area(m2) | 0.01 |
velocity | Velocity(m/s) | 20 |
How It Works
Aerodynamic Drag Force
Drag opposes the motion of an object through a fluid.
Formula
F_d = 0.5 Cd rho A v^2
Common Cd values: sphere 0.47, streamlined body 0.04, flat plate 1.28, car 0.25-0.45.
Worked Example
Sphere (Cd = 0.47) of cross-section 0.01 m2 in air at 20 m/s.
- 01F_d = 0.5 * 0.47 * 1.225 * 0.01 * 400
- 02F_d = 0.235 * 1.225 * 4
- 03F_d = 1.152 N
Frequently Asked Questions
Why does drag scale with velocity squared?
At moderate to high speeds, drag is dominated by inertial effects (pressure drag). The kinetic energy of deflected air grows as v^2, so the force does too.
What determines the drag coefficient?
Primarily the shape and surface roughness. Streamlined shapes direct flow smoothly with less separation, giving lower Cd.
Does Cd depend on Reynolds number?
Yes, especially for smooth objects. The drag crisis for a sphere occurs around Re = 500 000, where Cd drops sharply.
Ready to run the numbers?
Open Drag Coefficient Calculator