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Reynolds Number Calculator

Calculate the Reynolds number for a fluid flow and see whether it falls in the laminar, transitional, or turbulent range.

Reynolds number (Re)
99,600.8
Flow regime
Turbulent

Where this falls

6,000 falls in the "Turbulent" range.

How it works

The Reynolds number is a dimensionless ratio of inertial to viscous forces in a fluid flow: Re = (density × velocity × characteristic length) ÷ dynamic viscosity. Because it's a ratio of quantities that share the same units, it comes out as a pure number as long as density, velocity, length, and viscosity are all entered in consistent SI units (kg/m³, m/s, m, and Pa·s) — that's why this calculator doesn't offer a metric/imperial toggle: mixing systems on just some of the four inputs would silently break the ratio, and there's no single standard imperial unit for dynamic viscosity the way there is for length or weight.

For flow through a pipe, the widely used reference points are Re below 2,300 for laminar flow (smooth, layered, predictable), Re above 4,000 for turbulent flow (chaotic mixing), and the range in between as transitional, where either regime can occur depending on small disturbances. The default values above model water at roughly room temperature flowing through a small pipe.

FAQ

What counts as the 'characteristic length'?

It depends on the geometry being analyzed. For flow through a round pipe, it's the pipe's internal diameter — that's the convention this calculator's laminar/turbulent thresholds (2,300 and 4,000) are based on. For flow around an object, such as a sphere or an airfoil, engineers typically use that object's own characteristic dimension instead, and the transition thresholds differ.

Why does a higher Reynolds number mean more turbulent flow?

A high Reynolds number means inertial forces (the fluid's tendency to keep moving) dominate over viscous forces (the fluid's internal friction, which damps out disturbances). When viscosity can't smooth out small disturbances fast enough, they grow into the chaotic swirling and mixing that characterizes turbulent flow.

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