Reynolds Number Calculator
Re = ρ·V·D ÷ μ, with the laminar, transitional and turbulent classification.
What is Reynolds Number?
The Reynolds number is the ratio of inertial to viscous forces in a flow — a single dimensionless figure that predicts whether a flow glides in ordered layers or breaks up into eddies. It decides which friction factor applies, whether heat transfer is by conduction or mixing, and where a flow meter will be accurate. Below about 2300 in a pipe the flow is laminar; above about 4000 it is turbulent; in between it can be either and flips unpredictably.
How it works
Re is density × velocity × diameter ÷ dynamic viscosity, with everything in SI. The page also reports the kinematic viscosity (μ ÷ ρ), the number engineers quote for oils and air because it is independent of density. Fluid presets fill density and viscosity for water, sea water, air, SAE 30 oil, glycerin and petrol at 20 °C, since looking those up is most of the work. The thresholds are the internal-pipe-flow ones and the page says so — geometry changes them.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Realistic sample
Type or paste any realistic input — the tool computes the result locally and instantly.
Common mistakes
Expecting a server round-trip
Nothing is uploaded — every tool in this category runs entirely in your browser.