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Water Flow in Tubes and Reynolds Number

Reynolds number for clean cold water flow.

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Turbulent or laminar flow is determined by the dimensionless Reynolds Number which is important when analyzing fluid flow where there is a substantial velocity gradient (i.e. shear). The Reynolds number indicates the relative significance of the viscous effect compared to the inertia effect and the number is proportional to the inertial force divided by the viscous force.

The flow is

  • laminar when Re < 2300
  • transient when 2300 < Re < 4000
  • turbulent when 4000 < Re

Reynolds Number for one liter of water at approximately 20oC (68oF) flowing through pipes of different dimensions:

Pipe Size
(inches)11 1/223468101218
Reynolds number with
1 liter/min
835 550 420 280 210 140 105 85 70 46
Reynolds number with
1 gal/min
3180 2090 1600 1060 780 570 400 320 265 175

Note that the water viscosity varies with temperature.

  • the kinematic viscosity of water at 20oC - used to calculate the table above - is 1.004·10-6 m2/s
  • at 0oC the kinematic viscosity is 1.787·10-6 m2/s - the Reynolds values in the table above must be multiplicated with 1.004 / 1.787 = 0.56
  • at 100oC the kinematic viscosity is 0.29·10-6 m2/s - the values in the table above must be multiplicated with 1.004 / 0.29 = 3.46
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  • Engineering ToolBox, (2003). Water Flow in Tubes and Reynolds Number. [online] Available at: [Accessed Day Mo. Year].

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