Reynolds Number Calculator
The Reynolds number tells you whether flow in a pipe is laminar, transitional or turbulent, which affects pressure drop and the choice and accuracy of flow meters.
- Re = ρ × v × D ÷ μ
- = 998.2 × 2 × 0.10226 ÷ 0.001002 = 203,740
Flow regimes in round pipes
| Reynolds number | Regime |
|---|---|
| Below 2300 | Laminar |
| 2300 to 4000 | Transitional |
| Above 4000 | Turbulent |
Reynolds Number Formula
Where ρ is density (kg/m³), v is average velocity (m/s), D is the pipe inside diameter (m), μ is dynamic viscosity (Pa·s) and ν = μ/ρ is kinematic viscosity (m²/s). Re has no units.
Worked example
Water (998.2 kg/m³, 1.002 cP) at 2 m/s in a 102.26 mm pipe: Re = 998.2 × 2 × 0.10226 ÷ 0.001002 = 203,700, well into turbulent flow.
Frequently Asked Questions
What Reynolds number is turbulent flow?
In round pipes flow is normally turbulent above about 4000 and laminar below about 2300, with an unstable transition region in between.
Why do flow meters need a minimum Reynolds number?
Meters such as orifices, vortex and turbine meters are calibrated for a fully developed turbulent profile. At low Re their coefficient or K factor shifts, so manufacturers state a minimum Re.
How do I convert cP to cSt?
Divide dynamic viscosity in cP by density in g/cm³. Water at 20 °C is about 1.0 cP and 1.0 cSt.
Learn more on the blog
Coriolis flow meter uncertainty
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article