How this calculation works
Density, average speed, internal diameter and dynamic viscosity form a dimensionless ratio. Convert millimetres to metres and mPa·s to Pa·s before evaluating it.
Re = ρvD/μ; kinematic viscosity ν = μ/ρA worked example
At density 1,000 kg/m³, velocity 0.1 m/s, diameter 20 mm and dynamic viscosity 1 mPa·s, Re = 2,000.
Good to know
The displayed guide uses OpenStax’s approximate 2,000 and 3,000 thresholds for tube flow; actual transition depends on disturbances and geometry. The conditional 64/Re Darcy factor applies only to fully developed laminar circular-pipe flow.
Reference: OpenStax: viscosity, tube flow and Reynolds number (opens in a new tab) ↗
The ratio compares inertia with viscosity
Reynolds number is dimensionless: the units cancel in ρvD/μ. A larger speed, density or diameter raises it, while greater dynamic viscosity lowers it. These are relationships within a chosen pipe-flow model, not a universal label for every moving fluid.
At the default inputs, density 1,000 kg/m³, speed 0.1 m/s, diameter 0.02 m and viscosity 0.001 Pa·s produce Re = 2,000. Converting 1 mPa·s to 1 Pa·s by mistake would make the result one thousand times too small.
The Darcy Weisbach pressure loss calculator accepts a separately justified Darcy factor rather than inferring one from a transition label.
Use the guide as an approximate interpretation
This page quotes the approximate OpenStax tube-flow guide: below about 2,000 is typically laminar, above about 3,000 typically turbulent, and the intervening range is sensitive to disturbances. Different engineering conventions use other transition bands.
The table changes speed alone and keeps diameter and fluid properties fixed. A pipe flow calculator can supply average velocity from measured flow. For a real fluid, viscosity and density must match the relevant temperature and pressure.
Dynamic and kinematic viscosity are different inputs
Dynamic viscosity μ has units Pa·s. Kinematic viscosity ν = μ/ρ has units m²/s, often expressed in mm²/s. Entering a kinematic-viscosity number into the dynamic field does not preserve the Reynolds number.
The displayed 64/Re expression is conditional: it is a Darcy factor for fully developed laminar circular-pipe flow. The Darcy Weisbach pressure loss calculator requires a friction factor appropriate to its actual flow; this page does not choose a turbulent factor.
A few questions, answered
Does Re equal to 2,000 prove turbulent flow?
No. It lies at an approximate guide boundary; disturbances and the actual geometry matter.
Why is 64/Re undefined at zero speed?
Re is zero, so that expression divides by zero. There is no bulk flow in the entered model.
Can I use this formula for flow around a car?
External-flow Reynolds numbers use a suitable body length scale and different interpretation; this page specifically uses pipe diameter.