Calculate the Reynolds number from fluid density, velocity, characteristic length and viscosity, and see whether the flow is laminar or turbulent.
Last updated: November 8, 2026
The ratio of inertial to viscous forces
The Reynolds number (Re) is the ratio of inertial to viscous forces in a flow. It has no unit: • Re = ρ × v × L ÷ μ • ρ density (kg/m³), v velocity (m/s), L characteristic length (m), μ dynamic viscosity (Pa·s) Example: water at 20 °C (998 kg/m³, 1.0 mPa·s) flowing at 1 m/s through a pipe with a 5 cm inner diameter gives Re = 998 × 1 × 0.05 ÷ 0.001 = 49,900. The flow is turbulent. In pipes, Re below 2,300 is usually laminar, 2,300–4,000 transitional and above 4,000 turbulent. These limits are approximate, and flow over plates, spheres or in open channels uses different ones.
Common questions about the Reynolds number
In laminar flow the fluid moves in smooth layers. In turbulent flow eddies form, increasing mixing and friction losses.
Other tools you may find useful
Water at 20 °C ≈ 998 kg/m³.
Average flow speed.
Inner diameter for a pipe.
Water at 20 °C ≈ 1.0 mPa·s.