How it’s calculated
For an incompressible fluid filling a pipe, the volumetric flow rate equals the cross-sectional area times the average velocity (the continuity equation). Rearranging gives the diameter needed to carry a flow at a chosen velocity. Assumes a full circular pipe and the average (not centerline) velocity.
Example (OpenStax University Physics, Example 14.5): 0.500 L/s flows through a hose of radius 0.900 cm (D = 1.80 cm). v = 0.500 × 10⁻³ ÷ (π × 0.009²) = 1.96 m/s.
Designers often limit velocity to control noise, erosion, water hammer and pressure drop; check the pressure drop for the selected size as well.
Frequently asked questions
Should I use nominal or inside diameter?
Inside diameter. Nominal pipe size is only a label; wall thickness (schedule) changes the bore.
How do I convert gpm to L/s?
1 US gpm = 3.785411784 L ÷ 60 s = 0.06309 L/s.
Does this apply to gases?
Yes for volumetric flow at the actual (local) conditions, as long as density changes along the pipe are small.
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Sources
- University Physics Vol. 1, 14.5 Fluid Dynamics — OpenStax
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement — U.S. National Institute of Standards and Technology
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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