Reynolds Number Calculator
The Reynolds number is the dimensionless ratio of inertial to viscous forces in a flow. Enter the fluid properties below to compute Re and classify the regime — laminar, transitional or turbulent — for flow in a circular pipe.
Frequently asked questions
What Reynolds number is laminar?
For flow inside a circular pipe, Re below about 2300 is laminar, 2300–4000 is transitional, and above 4000 is fully turbulent. These thresholds are empirical and differ for other geometries such as flat plates or spheres.
What if I only know the kinematic viscosity?
Kinematic viscosity is ν = μ/ρ, so the formula becomes Re = vD/ν. Multiply ν by the density to recover μ for this calculator, or divide velocity × diameter by ν directly.
Why is the Reynolds number dimensionless?
The units of ρvD (kg·m⁻¹·s⁻¹) exactly cancel the units of μ (Pa·s = kg·m⁻¹·s⁻¹). Because Re is a pure number, two geometrically similar flows with the same Re behave identically — the basis of scale-model testing.