Reynolds Number Calculator

Regime thresholds

• Pipe flow: laminar Re < 2,300; transitional 2,300–4,000; turbulent Re > 4,000.
• Open-channel flow: laminar Re < 500; transitional 500–2,000; turbulent Re > 2,000. (Note the different thresholds — open channels go turbulent at lower Re because the free surface adds instability.)
• Flow over flat plates: laminar to ~5 × 10⁵ depending on edge condition.

Why it matters

The flow regime determines which friction equation applies. In laminar pipe flow, the Darcy friction factor is exactly f = 64/Re — independent of pipe roughness. In turbulent flow, roughness dominates and you need Colebrook or Swamee-Jain. In the transitional band, friction factor is unpredictable; engineering practice usually treats anything above 2,300 as effectively turbulent and uses the rough-pipe formulas conservatively.

Kinematic viscosity values

• Water at 40°F (4°C): 1.66 × 10⁻⁵ ft²/s (1.55 × 10⁻⁶ m²/s)
• Water at 60°F (15.6°C): 1.08 × 10⁻⁵ ft²/s (1.13 × 10⁻⁶ m²/s)
• Water at 80°F (26.7°C): 0.93 × 10⁻⁵ ft²/s (0.87 × 10⁻⁶ m²/s)
• Air at 60°F: 1.58 × 10⁻⁴ ft²/s (1.47 × 10⁻⁵ m²/s)
• Glycerine (pure): 7.0 × 10⁻³ ft²/s — orders of magnitude more viscous than water.

Reference: White, F.M. (2011). Fluid Mechanics (7th ed.). McGraw-Hill. Original concept: Reynolds, O. (1883). "An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous." Phil. Trans. Royal Soc. London, 174, 935–982.