Reynolds Number Calculator
Calculate the Reynolds number to determine if fluid flow is laminar or turbulent based on velocity, diameter, and viscosity.
Reynolds Number
The Reynolds number (Re) is a dimensionless quantity that predicts whether fluid flow will be laminar (smooth) or turbulent (chaotic).
Re = (ρ × v × D) / μ = (v × D) / ν
Where:
- ρ = fluid density (kg/m3)
- v = flow velocity (m/s)
- D = characteristic length/diameter (m)
- μ = dynamic viscosity (Pa·s)
- ν = kinematic viscosity (m2/s)
Flow regimes for pipe flow:
- Re < 2,300: Laminar flow — smooth, orderly layers
- 2,300 < Re < 4,000: Transitional — may be either
- Re > 4,000: Turbulent flow — chaotic, mixing
Common kinematic viscosities (at 20°C):
- Water: 1.004 × 10⁻⁶ m²/s
- Air: 1.516 × 10⁻⁵ m²/s
- Motor oil (SAE 30): ~3.0 × 10⁻⁴ m²/s
- Honey: ~2.0 × 10⁻³ m²/s
Applications: Pipe design, HVAC systems, aerodynamics, chemical engineering, blood flow analysis.