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How to use
Reynolds number Re is dimensionless and compares inertial with viscous effects in a flow. Use Re=ρvL/μ with density and dynamic viscosity, or Re=vL/ν with kinematic viscosity. The calculator also reports ν and a commonly used circular-pipe flow-regime guide.
- Choose dynamic-viscosity μ or kinematic-viscosity ν mode.
- Enter speed and characteristic length; dynamic mode also needs density.
- Calculate Re and read the pipe-flow guide.
How it is calculated
Example
Example: approximate water with ρ=1000kg/m³, v=1m/s, L=0.1m and μ=0.001Pa·s gives Re=100,000.
Important notes
The 2300 and 4000 thresholds shown here are common guidelines for internal circular-pipe flow, not universal boundaries for every geometry or external flow. Correct characteristic length and fluid properties at the actual temperature matter.
Worked examples and interpreting results
With assumed density 1000 kg/m³, dynamic viscosity 0.001 Pa·s, speed 0.01 m/s and diameter 0.1 m, Reynolds number is 1000 and has no unit.
| Case | Calculation | Result |
|---|---|---|
| Using dynamic viscosity | 1000 × 0.01 × 0.1 ÷ 0.001 | 1,000 |
| Using kinematic viscosity | 0.01 × 0.1 ÷ 0.000001 | 1,000 |
How to check the result
For a round pipe, use diameter as characteristic length. The calculator’s regime guide is for pipes, not automatically for a wing or open channel; the length definition matters.
Frequently asked questions
What are the units of Reynolds number?
It has no units; it is dimensionless.
How are μ and ν related?
Kinematic viscosity ν=μ/ρ.
Does Re>4000 always mean turbulent?
No. That threshold is only a common circular-pipe guide; transition depends on geometry and conditions.
How should characteristic length be chosen for Reynolds number?
It depends on geometry: pipe diameter for circular internal flow, distance from the leading edge for a flat plate, and hydraulic diameter for non-circular ducts. A wrong choice changes Re and its flow interpretation.