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Reynolds Number Calculator

Calculate the Reynolds number to determine whether flow is laminar, transitional or turbulent.

About Reynolds Number Calculator

The Reynolds number is the ratio of inertial to viscous forces, and it determines the flow regime: below about 2,300 flow is laminar, above 4,000 turbulent. Friction factor correlations differ entirely between the two regimes. Almost all industrial pipe flow is turbulent, which is why the laminar friction relationship is rarely the right one to use.

Formula

Reynolds Number

Re = ρ v L ÷ μ

Re
Ratio of inertial to viscous forces
μ
Dynamic viscosity

Worked example

With the default values below, Reynolds Number Calculator returns Reynolds Number of 200,000.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Fluid velocityminimum 0 m/s
2 m/s
Pipe diameter or characteristic lengthminimum 0 m
0.1 m
Fluid densityminimum 0.01 kg/m3
1,000 kg/m3
Dynamic viscosityminimum 0 Pa s
0.001 Pa s

Results

Reynolds Number
200,000.0
Flow Regime
Turbulent — mixed, chaotic flow
Kinematic Viscosity
0.00000100 m2/s
Velocity at Which Turbulence Begins
0.0400 m/s
Estimated Friction Factor
0.01494

Step by step

  1. 1Values used
  2. 2Reynolds Number
  3. 3Reynolds Number= 200,000.0
  4. 4Flow Regime= Turbulent — mixed, chaotic flow
  5. 5Kinematic Viscosity= 0.00000100 m2/s
  6. 6Velocity at Which Turbulence Begins= 0.0400 m/s
  7. 7Estimated Friction Factor= 0.01494

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 8 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Reynolds Number calculated?

Re = ρ v L ÷ μ The Reynolds number is the ratio of inertial to viscous forces, and it determines the flow regime: below about 2,300 flow is laminar, above 4,000 turbulent. Friction factor correlations differ entirely between the two regimes.

Why does Reynolds Number matter?

Almost all industrial pipe flow is turbulent, which is why the laminar friction relationship is rarely the right one to use.

What values do I need to enter?

This calculator takes 4 inputs: Fluid velocity, Pipe diameter or characteristic length, Fluid density, Dynamic viscosity. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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