Short answer
For a given flow rate and a selected mean velocity, the theoretical inside diameter is found from the relationship d = √(4Q/πv). The velocity input must be set by the person responsible for design; this tool does not by itself select a safe velocity or a hose model to purchase.
Technical explanation
When the relationship between circular cross-section and volumetric flow rate is solved in reverse, for Q in L/min and v in m/s, d(mm) = √(4000Q/(60πv)) is obtained. The result is a continuous geometric value; it is not a nominal size in a catalog or a connection standard. Pressure loss, viscosity, temperature, actual inside diameter and connection restrictions are evaluated separately in the final selection. This simple tool is not used for gas flow unless the pressure and temperature conditions are defined.
Topic-specific checkpoints
| Item | Description |
|---|---|
| Flow rate | Q in L/min |
| Target velocity | v with a defined design rationale |
| Theoretical diameter | Mathematical result in mm |
| Next check | Actual product and system requirements |
How to evaluate it in practice
For example, a flow rate of 10 L/min and a velocity of 2 m/s, chosen purely for illustration, give a theoretical inside diameter of approximately 10.30 mm. This example does not say that 2 m/s is suitable for all systems. After selecting a candidate product from the catalog, recalculate the velocity using the actual inside diameter. Do not automatically label the next larger nominal size as ‘safe’; the review of mechanical conditions and pressure loss must be completed. Both inputs must be positive and finite.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Is the first hose size above the theoretical diameter definitely the right choice?
No. The actual product size, pressure loss, connections and application conditions are verified separately.
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