Drag force calculator
Calculate drag from a known Cd, or recover Cd from a measured force. Use matching fluid conditions and reference area.
Your inputs
Updates as you editUse the density at your fluid's temperature and pressure.
Speed of the object relative to the fluid. Enter its magnitude.
Use the same area definition as your Cd source, such as frontal or wing area.
Dimensionless. Supply a measured or justified Cd for this flow and orientation.
Decimals and scientific notation are accepted. Unit changes preserve the physical value.
Optional. Record the object, orientation, Reynolds number, area definition or measurement source. This note travels with your result.
The example assumes ρ = 1.225 kg/m³, U = 10 m/s, A = 1 m² and Cd = 0.5. These are arithmetic inputs, not a coefficient recommendation for any object.
Calculated result
F = ½ρU²CdADrag force · F
N
- Dynamic pressure · q
- 61.25 Pa
- Drag area · CdA
- 0.5 m²
q = ½ρU², in pascals. 1 Pa = 1 N/m².
The product of coefficient and reference area, in m².
Inputs in SI units
ρ = 1.225 kg/m³ · U = 10 m/s · A = 1 m² · Cd = 0.5
What if the speed changes?
Hold density, Cd and reference area fixed. Drag changes with the square of speed.
×0.25 / ×1 / ×4 · of the current drag
This is a constant-Cd comparison. In real flow, Reynolds number or compressibility can change Cd as speed changes.
Cd depends on the body, Reynolds number, orientation, surface condition and compressibility. A coefficient measured for one setup may not apply to another. Match the fluid conditions and area convention to the source of your Cd.
The drag equation
F = ½ρU²CdA. Dynamic pressure q = ½ρU², so F = qCdA. To recover the coefficient, Cd = F / (qA).
Use ρ in kg/m³, U in m/s and A in m² to obtain F in newtons. Cd is dimensionless. The calculator converts the selected input units before applying the equation.
NASA explains that Cd combines the effects of shape, orientation and flow conditions. It also explains why a reported coefficient must include its reference area. NASA's drag equation.
A coefficient belongs to a set of conditions
Cd depends on the body, Reynolds number, orientation, surface condition and compressibility. A coefficient measured for one setup may not apply to another. Match the fluid conditions and area convention to the source of your Cd.
If you double the reference area for the same measured drag and flow, the reported Cd halves. The product CdA stays the same. Frontal area and wing area are different conventions.
The numerical input range is 0 or 10⁻¹⁰⁰ to 10¹⁰⁰ in SI units. Density must be positive. Primary SI results outside that range are rejected. This range is a numerical limit, not a claim of physical validity.
Return to the wind tunnel experiment
Compare shapes, orientations and wake patterns in the wind tunnel simulator. Its animation and relative pressure display do not provide a calibrated drag coefficient. They cannot supply a reliable Cd to this calculator.
Questions about drag and Cd
How do I calculate drag coefficient from force?
Use Cd = 2F / (ρU²A), with measured drag force and matching density, relative speed and reference area. Speed, area and density must be positive. A zero measured force then gives Cd = 0.
What happens at zero speed or zero area?
With a supplied finite Cd, the equation gives zero force at zero speed or zero reference area. Recovering Cd then requires division by zero, so it is undefined even when the measured force is also zero.
Can this calculator find Cd from an object's shape?
No. Supply a coefficient or measured drag for defined conditions. A shape label or a qualitative wind tunnel animation cannot determine a reliable engineering Cd.
Does doubling speed always quadruple drag?
The equation gives four times the force when speed doubles and density, Cd and area stay fixed. Real Cd can change with Reynolds number and compressibility, so the comparison is conditional.
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