Reynolds number calculator
The Reynolds number is the ratio of inertial to viscous forces and tells you the flow regime. Defaults are roughly water at 20 °C.
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How to use it
- Enter fluid density, flow velocity, inner diameter and dynamic viscosity.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Re = ρ v D ÷ μ. Laminar below 2300, transitional from 2300 to 4000, turbulent above 4000 (conventional limits for round pipes).
Example
Inputs: Fluid density = 998.2 kg/m³, Flow velocity = 1 m/s, Inner diameter = 50 mm, Dynamic viscosity = 1.002 mPa·s. Result: Reynolds number: 49,810.4; Flow regime: Turbulent.
Limits and caveats
- The 2300 and 4000 limits are approximate; real transition depends on roughness and inlet disturbances.
- Viscosity changes a lot with temperature, so enter its value at the working temperature.
Questions about this tool
What does the Reynolds number mean?
It is a dimensionless number comparing inertial and viscous forces. Low values mean smooth layered flow; high values mean eddies and turbulence.
How do I use it for a non-round duct?
Use the hydraulic diameter Dh = 4A ÷ P, where A is the section area and P the wetted perimeter.
What is the viscosity of water?
About 1.0 mPa·s at 20 °C, falling to about 0.55 mPa·s at 50 °C.