Reynolds Number Calculator

Find the Reynolds number for pipe flow to check whether it's laminar or turbulent.

Inputs

Fluid Density (kg/m³)

Water ≈ 1000 kg/m³, air ≈ 1.2 kg/m³.

Flow Velocity (m/s)

Pipe Diameter (m)

Dynamic Viscosity (Pa·s)

Water ≈ 0.001 Pa·s at 20°C, air ≈ 0.000018 Pa·s.

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Reynolds Number

100,000

Spark says

How it's calculated

Formula

Re = \dfrac{\rho V D}{\mu}

\rho: — Fluid density
V: — Flow velocity
D: — Pipe diameter
\mu: — Dynamic viscosity

What is the Reynolds Number Calculator?

The Reynolds number is a dimensionless ratio of inertial to viscous forces in a fluid flow — for pipe flow, Re below ~2,300 is typically laminar (smooth), above ~4,000 is turbulent, and between is a transitional zone.

How to use it

1 Enter the fluid's density and dynamic viscosity.
2 Enter the flow velocity and pipe diameter.

Worked examples

Water at 2 m/s in a 50mm pipe

Re = 100,000 — well into the turbulent regime.

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Limitations

•The 2,300/4,000 laminar/turbulent thresholds are for flow in a straight circular pipe — other geometries (open channels, flow around objects) use different characteristic length definitions and thresholds.

Frequently asked questions

Why does flow regime matter?

Laminar and turbulent flow have very different friction losses and mixing behavior — pipe sizing, pump selection and heat transfer calculations all depend on knowing which regime you're in.