About this tool
Reynolds Number Calculator is a free, in-browser tool that computes the dimensionless Reynolds number governing whether a flow is smooth (laminar) or chaotic (turbulent). Enter the fluid density, flow velocity, characteristic length and viscosity — dynamic or kinematic — or load a water or air preset. All math runs locally and nothing is uploaded.
With dynamic viscosity μ the number is Re = ρ·v·L ÷ μ; with kinematic viscosity ν it simplifies to Re = v·L ÷ ν, since ν = μ ÷ ρ. Here ρ is density, v is velocity and L is the characteristic length (for a pipe, its diameter). For pipe flow the tool classifies the result as laminar below 2300, transitional between 2300 and 4000, and turbulent above 4000.
Use it to predict flow regime in pipes, around objects or in channels, and to check the assumptions behind a pressure-drop or drag calculation. For example ρ = 1000, v = 2, L = 0.05 and μ = 0.001 give Re = 100000, firmly turbulent.
Frequently asked questions
What is the Reynolds number?
It is the ratio of inertial to viscous forces in a flow, Re = ρ·v·L ÷ μ. Being dimensionless, it predicts flow behaviour independently of the specific fluid or scale, which is why it is central to fluid dynamics.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity μ (Pa·s) measures a fluid's resistance to shear; kinematic viscosity ν (m²/s) is μ divided by density. With ν you can use the shorter Re = v·L ÷ ν, and the tool switches formulas automatically.
What do the laminar, transitional and turbulent ranges mean?
For pipe flow, Re below 2300 is laminar (orderly layers), 2300–4000 is transitional (unstable), and above 4000 is turbulent (mixing eddies). These thresholds are conventions for internal pipe flow and differ for other geometries.
Does this run privately?
Yes. The Reynolds number is calculated entirely in your browser; your flow parameters are never uploaded.
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