How this calculator works
A full circular pipe carries a volumetric flow equal to cross-sectional area multiplied by mean velocity. Use the internal diameter, since nominal pipe size and outside diameter do not determine the fluid area. This is a continuity calculation and does not estimate the pressure required or the velocity profile.
Formula
v = Q/A = 4Q/(pi D^2); Q = pi D^2 v/4; D = sqrt(4Q/(pi v)).
Worked example
A 0.2 m bore carrying 0.03141592654 m3/s has mean velocity 1 m/s. At twice that flow the same pipe carries 2 m/s.
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Frequently asked questions
Is this the centreline velocity?
No. It is the area-average velocity. Centreline speed depends on the flow profile.
Can the pipe be partly filled?
Use the flow-rate calculator for a circular segment; a full circle would overstate its area.
Why is zero velocity rejected in diameter mode?
A zero flow and zero velocity do not define a unique diameter, while positive flow at zero velocity requires unbounded area.