When is it useful?
Reynolds number compares the relative importance of inertia and viscosity. It helps compare flow inside a pipe or around an object, but the same value can mean something different if the geometry or reference length changes.
Formula and inputs
Enter density ρ in kg/m³, flow speed v in m/s, characteristic length L in m, and dynamic viscosity μ in Pa·s. For flow inside a circular pipe, L is usually its inner diameter. If you have kinematic viscosity ν in m²/s instead, use Re = vL/ν; do not enter ν as μ.
Open this calculatorWorked example
For an illustrative circular pipe, let ρ = 1000 kg/m³, v = 0.2 m/s, L = 0.01 m, and μ = 0.001 Pa·s. Then Re = 1000 × 0.2 × 0.01 ÷ 0.001 = 2000. The units cancel.
Interpreting the result
With mean velocity and inner diameter for this pipe example, 2000 is approximately in the laminar-flow range. The calculator’s 2300 and 4000 boundaries are simplified guidance for internal pipe flow.
Limits and cautions
Do not carry the pipe thresholds over to a pipe entrance, bend, rough wall, or another geometry unchanged. Use viscosity measured or tabulated at the relevant temperature.
Formula and concept reference: NASA Glenn — Reynolds Number. We created the sample inputs and calculations for this guide.