Reynolds Number Calculator
Calculate Reynolds number from velocity, diameter and viscosity to check laminar or turbulent flow with this free fluid dynamics calculator.
Enter Values
Minimum 0
Minimum 0
How to use this calculator
- Enter fluid density, velocity, characteristic length and dynamic viscosity.
- Read the Reynolds number.
How it works
Re = (density × velocity × length) ÷ dynamic viscosity.
Worked example
Water in a 50 mm pipe at 2 m/s. Re = 100,000 (turbulent).
Common mistakes
- Using kinematic viscosity where dynamic (Pa·s) is required.
- Picking the wrong characteristic length (pipe diameter for internal flow).
Frequently asked questions
What is the Reynolds number?
A dimensionless ratio of inertial to viscous forces, Re = ρvD ÷ μ, that predicts whether flow is laminar or turbulent.
What Reynolds number means turbulent flow?
In a pipe, flow is generally laminar below about 2,000, transitional to around 4,000, and turbulent above that.
What's laminar vs turbulent?
In pipes, Re below ~2300 is smooth laminar flow; above ~4000 it's turbulent, with a transitional zone between.
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Cite this tool
Zerdly. (2026). Reynolds Number Calculator. Retrieved from https://www.zerdly.com/tools/reynolds-number-calculator/
Tip: Enter any known values to calculate the remaining results.
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