Reynolds Number Calculator
Calculate Reynolds number from density, velocity, length and dynamic viscosity.
Reynolds Number Formula
Re = ρvL/μ.
What Is Reynolds Number?
Reynolds number is a dimensionless ratio that compares inertial effects with viscous effects in fluid flow. It is widely used to help characterize whether a flow is likely to be laminar, transitional or turbulent.
Example Calculation
For water with density 1,000 kg/m³, velocity 1 m/s, characteristic length 0.02 m and dynamic viscosity 0.001 Pa·s, Re = 1,000 × 1 × 0.02 / 0.001 = 20,000, typically indicating turbulent pipe flow.
Factors and Interpretation
Higher density, velocity or characteristic length increases Reynolds number, while higher viscosity decreases it. The calculator labels pipe-flow regimes using common thresholds near 2,300 and 4,000, but transition criteria vary with geometry, disturbances and application.
Frequently Asked Questions
Does Reynolds number have units?
No. It is dimensionless.
What characteristic length should I use for a pipe?
For circular internal pipe flow, the pipe diameter is commonly used.
Is Re above 4,000 always turbulent?
It is a common pipe-flow guideline, not a universal threshold for every geometry.
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Formula and Practical Limits
Reynolds number compares inertial and viscous effects using density, velocity, characteristic length and dynamic viscosity. Flow-regime thresholds depend on geometry and conditions; familiar pipe-flow thresholds should not be applied blindly to every system.