BIZOMAT MAKEOS · Hydraulics
Liquid Pipe Velocity and Reynolds Number
What velocity and flow regime follow from diameter, flow and viscosity?
Calculate mean velocity and Reynolds number for a full circular pipe carrying a single-phase Newtonian liquid.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Internal diameter | mm | Actual internal bore, not nominal pipe size. |
| Liquid flow | L/min | Volumetric flow at operating conditions. |
| Kinematic viscosity | mm²/s | Fluid viscosity at the operating temperature. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Internal diameter
- What it is
- The inside diameter of the pipe or tube the liquid flows through.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For a given flow, velocity falls with the square of bore and Reynolds number falls in proportion to bore.
- Get it right
- Use the real bore for the pipe schedule or tube wall, not the nominal pipe size.
What this means: Liquid flow
- What it is
- The volume of liquid passing through the pipe each minute at operating conditions.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0 L/min to 1000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Velocity and Reynolds number are directly proportional to flow.
- Get it right
- Flow in m³/h converts to L/min by multiplying by 1000 and dividing by 60.
What this means: Kinematic viscosity
- What it is
- The kinematic viscosity of the liquid at the operating temperature: its dynamic viscosity divided by density. 1 mm²/s equals 1 cSt.
- Units and format
- Enter a number in mm²/s: mm²/s. The form accepts 0.000001 mm²/s to 1000000 mm²/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm²/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Reynolds number is inversely proportional to viscosity.
- Get it right
- Do not enter dynamic viscosity in mPa·s; divide it by density in g/cm³ first. Oils thin markedly when hot.
Method and formulas
- Velocity = (flow / 60000) ÷ (π × (diameter / 1000)² / 4), in m/s.
- Re = velocity × (diameter / 1000) ÷ (kinematic viscosity × 10⁻⁶).
- The guide bands are Re < 2000, 2000–4000 and Re > 4000; real transition depends on disturbances. Example: 25 mm, 20 L/min and 1 mm²/s gives 0.679061 m/s and Re = 16976.53.
Limitations
- Not for compressed gas, open channels, two-phase flow or non-Newtonian fluids.
- No pressure drop, cavitation, erosion or suitable-velocity approval is inferred.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- pipe velocity
- Reynolds
- liquid flow
Who it is for
- Hydraulic engineers
- Process engineers
What you get
- Mean velocity
- Reynolds number
- Regime indication
Interpreting the result
- Use Reynolds number to select a suitable friction model and include fittings and roughness in pressure-drop work.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 365–366; CD pp. 3022–3024. Topic reference; no source tables or recommended settings are reproduced.
- US Department of Energy, DOE-HDBK-1012/3-92, Fluid Flow, pp. 9–11 and 19–20 — https://www.energy.gov/sites/default/files/2026-04/DOE-HDBK-1012-92_VOL3.pdf
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
What a flowmeter measures
In Flow Measurement by the Numbers
What it covers: Volume, mass and standard volume flow and how to convert between them, mean velocity and Reynolds number, why temperature changes a litre, and a hot water recipe that came out 2.6% short.
Method explained here · BIZOMAT original book
The Reynolds conflict and viscous scale effects
In Scale Models by the Numbers
What it covers: See where viscosity acts in a Froude model, judge Reynolds independence, raise the model Reynolds number with temperature and flow, and tell conservative scale effects from unsafe ones.
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