How to read Reynolds number and flow regime
Reynolds number is the dimensionless ratio of inertial to viscous forces, and it decides whether flow inside a pipe is laminar or turbulent. The base equation is Re = ρVD/μ. With density in lb/ft³, velocity in ft/s, diameter in inches and viscosity in centipoise, the unit conversions collapse into a single factor, so this page uses Re = 124.0 × ρVD/μ. Water at 68°F (62.3 lb/ft³, 1.002 cP) moving 6 ft/s through a 2 in pipe gives Re near 92,500, firmly turbulent.
| Reynolds number | Regime | Behavior |
|---|---|---|
| Below 2,300 | Laminar | Smooth layers, f = 64/Re |
| 2,300 to 4,000 | Transitional | Switches back and forth, hard to predict |
| Above 4,000 | Turbulent | Strong mixing, better heat transfer |
Flow regime is the starting point for both pressure drop and heat transfer work. In laminar flow the friction factor depends on Reynolds number alone, while turbulent flow also responds to internal roughness. The turbulent factor shown here is the Blasius approximation for smooth pipe, so old rough piping or any case above Re 100,000 should be checked against the Colebrook equation or a Moody chart. For ducts that are not round, substitute the hydraulic diameter for D. Have a mechanical engineer or the inspection authority confirm the final design.
Frequently Asked Questions
It is a convention that different references place between 2,000 and 2,320. Vibration or a disturbed inlet can trip turbulence at lower values, so treat results near the boundary with margin.
Water is about 1.002 cP at 68°F and about 0.653 cP at 104°F, so temperature matters a lot. For oils and chemicals, take the value at your operating temperature from the property table or safety data sheet.