Reynolds Number Calculator
Calculate the Reynolds number for pipe or duct flow and see whether it is laminar, transitional or turbulent. Enter a velocity or a volumetric flow rate, pick a fluid preset (water, air, oil, glycerin) or your own density and viscosity, and work in metric or imperial units.
Input
Flow
Flow rate is converted to a mean velocity using the cross-section, so it needs a pipe or duct geometry.
Geometry
Internal bore, not the nominal or outside diameter.
Fluid
Presets are at 1 atm. Viscosity is strongly temperature-dependent — pick the closest.
Output
| Property | Value |
|---|---|
| No data yet | |
| Regime | Reynolds Range | Behaviour | This Flow |
|---|---|---|---|
| No data yet | |||
The conventional transition bands for flow in a closed conduit.
Guides
The Reynolds number is the dimensionless ratio of inertial to viscous forces in a flowing fluid. It is the single number that tells you whether flow in a pipe or duct is smooth and orderly (laminar), chaotic (turbulent), or sitting unpredictably between the two — and it decides which friction-factor correlation, heat-transfer relation and pressure-drop method you are allowed to use.
This calculator works it out from the velocity (or the flow rate), the cross-section, and the fluid, then classifies the regime and shows every intermediate value in SI so you can check the arithmetic.
How to use it
- Choose whether you know the mean velocity or the volumetric flow rate. Flow rate is divided by the cross-sectional area to get the mean velocity, so it needs a real geometry.
- Pick the cross-section: a circular pipe (enter the inside bore), a rectangular duct (width × height), or a hydraulic diameter you have already worked out.
- Pick a fluid preset — water, seawater, air, ethanol, SAE 30 oil, glycerin or mercury at a stated temperature — or choose Custom fluid and type your own density and viscosity in centipoise, pascal-seconds or centistokes.
The result gives the Reynolds number, the regime, and a breakdown row for every converted quantity: velocity in m/s, hydraulic diameter in m, density, dynamic viscosity, kinematic viscosity, flow area and volumetric flow.
The formula
Re = ρVD / μ, where ρ is density (kg/m³), V the mean velocity (m/s), D the hydraulic diameter (m) and μ the dynamic viscosity (Pa·s). Equivalently Re = VD / ν using kinematic viscosity ν = μ/ρ.
For a full circular pipe the hydraulic diameter is simply the inside diameter. For a rectangular duct of width w and height h it is Dh = 4A/P = 2wh / (w + h) — the calculator applies this for you.
Flow regimes
| Regime | Range | What it means |
|---|---|---|
| Laminar | Re < 2300 | Smooth layers, friction factor f = 64/Re |
| Transitional | 2300 – 4000 | Unstable; avoid designing in this band |
| Turbulent | Re > 4000 | Chaotic mixing; use Colebrook/Moody |
These thresholds are the conventional ones for flow inside a closed conduit. Flow over a flat plate or around a sphere transitions at completely different values.
Which viscosity should I enter?
Dynamic viscosity (centipoise or Pa·s) is what the formula wants. If your data sheet only quotes kinematic viscosity in centistokes — common for lubricating oils — select cSt and the calculator multiplies by the density for you. Water at 20 °C is 1.002 cP; that is the reference everything else is compared against.
Why does temperature matter so much?
Viscosity, not density, drives the Reynolds number, and it falls steeply with temperature. Water at 60 °C is less than half as viscous as at 20 °C, which more than doubles Re for the same pipe and velocity. Always use properties at the actual operating temperature — the presets state theirs.
Does this give me the pressure drop?
No. The Reynolds number is the input to a pressure-drop calculation, not the result of one. Once you have Re you pick a friction factor (64/Re when laminar, Colebrook or a Moody chart when turbulent) and feed it into Darcy–Weisbach.
Privacy
Everything runs in your browser. No velocities, dimensions or fluid properties are sent anywhere.
Related tools
Working in mixed units? The Pressure Converter and Speed Converter handle the unit changes this calculator does not, and the general Unit Converter covers volume, area and length. For fluid properties that depend on temperature, the Temperature Converter will get you between °C, °F and K first.