Drag Equation Calculator

The drag force is given by the drag equation:

$$F_d = \frac{1}{2} C_d \rho A v^2$$

where:

• F_d — drag force (N)
• C_d — drag coefficient (dimensionless)
• ρ — fluid density (kg/m³)
• A — reference area (m²), typically frontal projected area
• v — velocity relative to the fluid (m/s)

The term ½ρv² is the dynamic pressure (q), representing the kinetic energy per unit volume of the flow:

$$q = \frac{1}{2}\rho v^2$$

The power required to overcome drag at constant velocity:

$$P = F_d \cdot v = \frac{1}{2} C_d \rho A v^3$$

Note the cubic dependence on velocity for power — doubling speed requires 8× the power to overcome drag alone.

Typical drag coefficients:

• Sphere: C_d ≈ 0.47
• Sedan car: C_d ≈ 0.25–0.35
• SUV/Truck: C_d ≈ 0.35–0.45
• Streamlined body: C_d ≈ 0.04–0.10
• Flat plate (perpendicular): C_d ≈ 1.17
• Cylinder (crossflow): C_d ≈ 1.0–1.2
• Bicycle + rider: C_d ≈ 0.9 (upright), 0.5 (drops)

The drag coefficient depends on the body shape, surface roughness, and Reynolds number. At very low Re (Stokes flow), C_d = 24/Re for a sphere. At high Re, C_d becomes approximately constant — the regime where this equation is most useful.

The drag force increases with the square of velocity, meaning aerodynamic drag dominates at high speeds. The power output shows the energy rate needed just to overcome drag — at highway speeds, aerodynamic drag typically accounts for 50–70% of a car's total resistance. The dynamic pressure provides a useful reference for comparing drag across different conditions.