Reynolds number
A regime screen, not a friction-factor lookup.
Method
Re = ρvD/μ. Worked case: water, 1.13 m/s, 150 mm, 0.001 Pa·s is about 169,500.
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Logic: how this figure is built.
How to read a Reynolds number
Reynolds number is ρvD/μ. Water at about 1.13 m/s in a 150 mm bore, with viscosity 0.001 Pa·s, is about 169,500. That is turbulent on the usual pipe threshold near 4,000. The page reports the number. It does not pick a friction factor.
Diameter is the bore. Velocity is the mean velocity, which is discharge over area. A centre-line velocity is higher. Viscosity depends on temperature. Cold water is more viscous than the 0.001 figure.
A Reynolds number does not say the pipe is acceptable. It says which flow regime the straight-pipe loss model should assume. Laminar and turbulent losses are calculated differently.
Use it beside the pipe-velocity page. It is not a pressure-drop result.
- Related: pipe velocity
- fluids logic
Last checked 3 October 2026. See the common-calculations note and the disclaimer.