Reynolds Number Calculator
Enter flow velocity, a characteristic length such as pipe diameter, and the fluid viscosity. The calculator returns the Reynolds number and the flow regime.
How to find the Reynolds number
- Enter the flow velocity in metres per second.
- Enter the characteristic length (pipe diameter) in metres.
- Give the kinematic viscosity, or the density and dynamic viscosity.
- Read the Reynolds number and whether the flow is laminar or turbulent.
Examples
Water in a 50 mm pipe
v = 2 m/s, D = 0.05 m, density 1000, viscosity 0.001 Pa s
Re = 100000 (turbulent)
Slow flow, same pipe
v = 0.01 m/s, D = 0.05 m, kinematic viscosity 1e-6
Re = 500 (laminar)
Frequently asked questions
What is the Reynolds number formula?
Re = density x velocity x length / dynamic viscosity. Using kinematic viscosity it simplifies to Re = velocity x length / kinematic viscosity.
When is flow laminar or turbulent?
For pipe flow, Re below 2300 is laminar, 2300 to 4000 is transitional, and above 4000 is turbulent. This calculator uses those thresholds.
What is the characteristic length?
It is the length scale of the flow. For a full round pipe it is the inside diameter. For other shapes use the hydraulic diameter.
Do I need density and viscosity, or just one number?
Either works. Provide kinematic viscosity on its own, or supply density and dynamic viscosity and the tool divides them for you.
Why is the Reynolds number dimensionless?
The units of density, velocity, length and viscosity cancel, so the Reynolds number is a pure ratio of inertial to viscous forces with no units.
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