Reynolds number flow regimes
The Reynolds number thresholds that separate laminar, transitional, and turbulent flow in a round pipe. The Reynolds number is a dimensionless ratio that predicts whether flow will be smooth or chaotic, which drives friction, pressure loss, and heat transfer.
Flow regimes in a round pipe
| Reynolds number | Flow regime | Behavior |
|---|---|---|
| Below 2300 | Laminar | Smooth, orderly layers; viscosity dominates |
| 2300 to 4000 | Transitional | Unstable, flickering between laminar and turbulent |
| Above 4000 | Turbulent | Chaotic mixing; inertia dominates |
Reynolds number (Re) is the ratio of inertial to viscous forces: Re = ρvD/μ = vD/ν, where v is velocity, D is the pipe diameter, ρ is density, μ is dynamic viscosity, and ν is kinematic viscosity. It is dimensionless. The thresholds above are for flow in a round pipe; other geometries shift them.
Want to compute Re for your flow?
The Reynolds Number Calculator finds Re from velocity, diameter, and fluid properties, and tells you the regime.
Why the flow regime matters
Laminar flow is smooth and predictable, with low friction and little mixing, typical of slow flow, thick fluids, or small tubes. Turbulent flow mixes aggressively, raises friction and heat transfer, and is the norm for water in everyday pipes. Pressure drop, pump sizing, and heat transfer all depend on which regime you are in.
Reading the thresholds
For a round pipe, flow is reliably laminar below about Re 2300 and reliably turbulent above about 4000. Between those lies a transitional band where the flow flickers between the two and is hard to predict. Designers usually keep clear of the transition by sizing pipes for solidly laminar or solidly turbulent flow.
FAQ
What Reynolds number is turbulent?
In a round pipe, flow is generally turbulent above a Reynolds number of about 4000 and laminar below about 2300. Between those values it is transitional.
What does the Reynolds number tell you?
It compares inertial to viscous forces and predicts whether flow will be smooth (laminar) or chaotic (turbulent), which sets friction and pressure loss.
Is the Reynolds number unitless?
Yes. It is a dimensionless ratio, the same in any consistent unit system as long as the inputs are consistent.