About the Reynolds Number Calculator
What is the Reynolds Number Calculator?
The Reynolds number compares inertial and viscous forces in a flowing fluid. It tells you whether flow is smooth and layered (laminar) or chaotic (turbulent), which changes friction and heat transfer.
Choose a fluid – water, air or hydraulic oil – or enter a custom kinematic viscosity.
How to use the Reynolds Number Calculator
- Flow Velocity (m/s) (sample: 1.5)
- Characteristic Length – pipe diameter (m) (sample: 0.05)
- Fluid (sample: Water at 20 °C (1.004 cSt))
- Custom Kinematic Viscosity (cSt = mm²/s) (sample: 1)
- Press Calculate Reynolds Number (results also refresh as you type). The result card shows the Reynolds Number, along with Flow regime (pipe flow), Kinematic viscosity used and Velocity for Re = 2,300.
Formula used
Re = v × D ÷ ν
- ν = kinematic viscosity (1 cSt = 10⁻⁶ m²/s)
- Pipe flow = laminar below 2,300, turbulent above 4,000
Viscosity falls sharply as liquids heat up, raising the Reynolds number.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Flow Velocity (m/s) | 1.5 |
| Characteristic Length – pipe diameter (m) | 0.05 |
| Fluid | Water at 20 °C (1.004 cSt) |
| Custom Kinematic Viscosity (cSt = mm²/s) | 1 |
| Result | Value |
|---|---|
| Reynolds Number | 74,701 |
| Flow regime (pipe flow) | Turbulent |
| Kinematic viscosity used | 1.004 cSt |
| Velocity for Re = 2,300 | 0.0462 m/s |
Tips for using the Reynolds Number Calculator
- Use the hydraulic diameter (4 × area ÷ perimeter) for non-circular ducts.
- Hydraulic oil lines often run laminar, especially when cold.