Reynolds Number Calculator
Laminar, transitional or turbulent.
Work out Reynolds Number. Laminar, transitional or turbulent. Free, with no account and nothing to download.
Pipe diameter for internal flow, or the object's length for flow past it.
Water 0.001, air 1.8e-5, olive oil 0.08, honey about 10.
Reynolds number
50,000
Turbulent — chaotic mixing, much higher friction losses
Re is the ratio of inertial to viscous forces, and it is the single number deciding whether flow is orderly or chaotic. Below about 2,300 in a pipe it is laminar; above roughly 4,000 turbulent; between them it depends on how smooth the entry was. Because it is dimensionless, a scale model in a wind tunnel behaves like the full-size aircraft as long as the Reynolds numbers match. That similarity is what makes model testing meaningful rather than merely suggestive.
How the Reynolds Number Calculator works
Enter density, speed, a characteristic length and viscosity to get the Reynolds number and which flow regime you are in. It is dimensionless, which is what lets a wind-tunnel model stand in for a full-size aircraft.
Also known as: laminar or turbulent flow calculator · flow regime calculator · critical reynolds number · reynolds number formula
Frequently asked questions
What is the Reynolds number?
Re = ρvL/μ — the ratio of inertial to viscous forces. It decides whether a flow stays in orderly layers or breaks into chaotic mixing.
What counts as laminar or turbulent?
In a pipe, below about 2,300 is laminar and above roughly 4,000 is turbulent. Between them it is transitional and depends on how smooth the entry was — the same setup can go either way.
What is the characteristic length?
Pipe diameter for internal flow, or the streamwise length of the object for flow past something. Getting it wrong shifts the answer by whatever factor you used, so state which convention you are on.
Why does it matter for scale models?
Because it is dimensionless, two flows with the same Reynolds number behave the same way regardless of size. Matching it is what makes wind-tunnel testing meaningful rather than merely suggestive.
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