Calculates the Reynolds number, a dimensionless quantity that predicts whether a fluid flow will be smooth (laminar) or chaotic (turbulent).
How it works
Fluid density, velocity, and a characteristic length are multiplied together, then divided by the fluid’s dynamic viscosity.
What this does not include
This gives the general flow-regime guideline for pipe flow — the exact laminar/turbulent transition point varies somewhat by geometry and flow conditions.
How to use this calculator
- Enter the fluid’s density, flow velocity, a characteristic length (like pipe diameter), and viscosity.
A worked example
Water (density 1,000 kg/m³) flowing at 2 m/s through a 0.05 m diameter pipe, viscosity 0.001 Pa·s: Reynolds number = 100,000 — well into the turbulent regime.
Air (density 1.225 kg/m³) at 10 m/s over a 1 m diameter object, viscosity 0.0000181 Pa·s: Reynolds number = 676,795.58.
What the variables mean
| Variable | Meaning |
|---|---|
| Density | Fluid density |
| Velocity | Flow speed |
| Diameter | Characteristic length — pipe diameter or object size |
| Viscosity | Fluid’s resistance to flow |
Edge cases worth knowing
A low Reynolds number means smooth, laminar flow; a high one means chaotic, turbulent flow. The transition happens around 2,000–4,000 for pipe flow — well below both examples above, which is why both land solidly in turbulent territory.
Zero viscosity makes the Reynolds number infinite, which has no physical meaning for a real fluid, so the calculator declines to show a result.
Frequently asked questions
What does a low Reynolds number mean?
Flow tends to be smooth and orderly (laminar), dominated by viscous forces — think honey flowing slowly rather than water rushing.
Why is 2,300 often cited as the laminar-to-turbulent threshold in pipes?
It’s a commonly used rule-of-thumb transition point for flow in a circular pipe — above roughly 4,000, flow is reliably turbulent, with a transitional zone in between.
What’s a “characteristic length”?
A representative dimension of the flow geometry — commonly a pipe’s inner diameter for internal pipe flow, or an object’s length for flow around it.
