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Reynolds Number Calculator
Calculate Reynolds number from velocity, characteristic length, and fluid viscosity, then compare the result with laminar, transitional, or turbulent ranges.
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Math & Science
Reynolds Number Calculator
Frontend preview — no upload or external service.
Reynolds number
Water at 20°C, 1.8 m/s through a 50 mm pipe: Re ≈ 89,600 — firmly turbulent, well above the 4,000 threshold where pipe flow ceases to be laminar.
How the Reynolds Number Calculator works
- Enter the flow velocity and the characteristic length, which is the internal diameter for a pipe.
- Select the fluid, or enter kinematic viscosity directly for anything unusual.
- Read the regime, remembering the thresholds are approximate and depend on geometry and how disturbed the flow is.
The method
Reynolds number is the ratio of inertial to viscous forces, and it is dimensionless — which is what makes it comparable across scales and fluids.
Re = ρvL/μ = vL/ν, where ν is kinematic viscosity
For water at 20°C, ν ≈ 1.004×10⁻⁶ m²/s, so 1.8 × 0.05 / 1.004e-6 ≈ 89,600.
FAQ
What are the laminar and turbulent thresholds?
For pipe flow, roughly below 2,300 is laminar, above 4,000 is turbulent, and between them is a transitional region that is genuinely unpredictable. Other geometries — flow over a plate, around a sphere — have completely different thresholds.
Why is Reynolds number dimensionless?
Because it is a ratio of two forces, so the units cancel. That is what allows a scale model in a wind tunnel to represent a full-size aircraft: match the Reynolds number and the flow behaves similarly.
What is the characteristic length?
Whatever the standard length scale is for the geometry — internal diameter for a pipe, chord for an aerofoil, distance from the leading edge for a flat plate. Using the wrong one makes the number incomparable.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Fluid properties built in | ✓ | Look-up | ✓ |
| Regime thresholds explained | ✓ | ✗ | Sometimes |
| No simulation software | ✓ | ✓ | ✗ |
| Works offline once loaded | ✓ | ✓ | ✗ |
Reynolds Number Calculator names the characteristic length for each geometry, since using the wrong length makes the number meaningless to compare.