Calculator · Fluid Mechanics
Reynolds Number Calculator
The Reynolds number compares inertia with viscosity for internal flow. It uses the speed magnitude and the hydraulic diameter, so a reversed flow never makes it negative.
Inputs
The magnitude is used; a reverse flow gives the same Reynolds number.
For a full circular pipe this is simply the internal diameter.
Water at 20 °C is about 1×10⁻⁶ m²/s.
Results
Reynolds number Re
Conventional regime: Turbulent.
| Mean speed V | 1 m/s |
|---|---|
| Hydraulic diameter D_h | 0.1 m |
| Kinematic viscosity ν | 1.0000e-6 m²/s |
| Reynolds number Re | 100000 |
| Flow regime | Turbulent |
Assumptions
- · Internal flow characterised by the hydraulic diameter.
Explore in the Lab
The Lab derives the same Reynolds number for every segment of a series.
Technical reference
Re = ρ |V| D_h / μ
Re = |V| D_h / ν
ν = μ / ρ
- Re
- Reynolds number[-]
- V
- mean speed of the flow[m/s]
- D_h
- hydraulic diameter of the section[m]
- ρ
- density of the fluid[kg/m³]
- μ
- dynamic viscosity[Pa·s]
- ν
- kinematic viscosity[m²/s]
Re < 2300 laminar, 2300 ≤ Re < 4000 transitional, Re ≥ 4000 turbulent. These are conventional engineering boundaries for internal pipe flow, not exact physical discontinuities.
The characteristic length for internal flow is the hydraulic diameter. External flow uses a different characteristic length, and that is a separate calculation.
Reference case: ν = 1×10⁻⁶ m²/s, V = 1 m/s and D = 0.1 m give Re = 100000.