Calculator · Fluid Mechanics
Drag Force Calculator
The drag on a body in a stream follows from its dynamic pressure, its reference area and a drag coefficient that comes from measurement. The coefficient is yours to supply: nothing here estimates it from a shape, a Reynolds number or a turbulence state.
Inputs
The speed of the body relative to the fluid far from it.
A coefficient is only meaningful with the area it was measured against.
Results
Drag force F_D = ½ ρ V_rel² C_D A_ref
A magnitude. With no stated direction of relative motion, none is invented.
| Fluid density ρ | 1.225 kg/m³ |
|---|---|
| Relative speed V_rel | 20 m/s |
| Dynamic pressure ½ρV² | 245 Pa |
| Drag coefficient C_D | 1 |
| Reference area A_ref | 0.5 m² |
| Drag force F_D | 122.5 N |
Assumptions
- · Steady incompressible flow with a prescribed drag coefficient.
- · The drag coefficient is supplied, not predicted from the Reynolds number.
Technical reference
F_D = ½ ρ V_rel² C_D A_ref
q = ½ ρ V_rel²
- F_D
- drag force magnitude on the body[N]
- V_rel
- speed of the body relative to the fluid[m/s]
- C_D
- prescribed drag coefficient[-]
- A_ref
- reference area the coefficient refers to[m²]
A zero relative speed gives exactly zero drag. A negative area, density or coefficient is not a valid state and is refused.
Lift, boundary layers and any prediction of C_D from geometry or Reynolds number are outside this version.
Reference case: ρ = 1.225 kg/m³, V = 20 m/s, C_D = 1, A = 0.5 m² gives F_D = 122.5 N.