Short answer
In a circular cross-section, mean flow velocity is calculated by dividing the volumetric flow rate by the actual flow cross-section: v = Q/A. This result is the mean velocity; on its own, it does not determine pressure loss, noise or suitable hose selection.
Technical explanation
For a circular, full cross-section, the area is A = πd²/4. When flow rate is in L/min, actual inside diameter in mm and velocity in m/s, the unit conversions give v = 4000Q/(60πd²). The diameter here is not the outside diameter or a nominal thread size. For gases, because the pressure and temperature to which the volumetric flow rate refers are critical, this simple tool must not be used as a compressible flow solution.
Topic-specific checkpoints
| Item | Description |
|---|---|
| Input Q | Volumetric flow rate in L/min |
| Input d | Actual inside diameter in mm |
| Formula | v = 4000Q / (60πd²) |
| Result | Cross-sectional mean velocity in m/s |
How to evaluate it in practice
For example, if the actual inside diameter is 10 mm and the volumetric flow rate is 10 L/min, the mean velocity is approximately 2.122 m/s. This value is a calculation example, not a recommended velocity limit. The flow rate must be zero or positive and the inside diameter positive. At zero flow, the velocity is zero; negative and invalid inputs are rejected. The inside diameter tolerance and narrow cross-sections in the connections are evaluated separately. The tool result must not be automatically converted into a pressure loss or a safe product recommendation from the catalog.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Can the outside diameter of the hose be used in the velocity calculation?
No. The actual inside diameter that determines the flow cross-section is required; calculating with the outside diameter gives a wrong result.
Source coverage
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S021
Selecting a hose assembly? Remember STAMPED Swagelok Bangalore · accessed 2026-10-19 Size; Temperature; Application; Material; Pressure; End Connections; Delivery
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S023
NIST Guide to the SI, Appendix B.8 NIST · accessed 2026-10-19 Alphabetical entries: bar, inch, pound-force per square inch