How this calculator works
Timed collection uses volume divided by elapsed time. Pipe mode uses the wetted cross-section multiplied by the average liquid velocity, not the velocity at one point. A partly full round pipe uses exact circular-segment area, including depths above the centreline. Reverse depth is found by a bounded numerical search. The tool does not infer velocity from slope, pressure or roughness; those require a hydraulic model.
Formula
Q = V/t = A x mean velocity; L/min = Q(m3/s) x 60000; Acircle = pi D^2/4; Arectangle = width x filled height; Asegment = r^2 acos((r-h)/r) - (r-h)sqrt(2rh-h^2).
Worked example
A collected volume of 120 L in 2 minutes gives 60 L/min. A 2 m internal-diameter pipe half full has wetted area pi/2 = 1.570796 m2; at 1 m/s mean velocity its flow is 1.570796 m3/s.
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Frequently asked questions
Does a partially filled pipe need a separate above-half option?
No. Enter the liquid depth from the bottom. The segment calculation automatically handles below-half, half-full and above-half conditions.
Is surface velocity the average velocity?
Usually not. The cross-section has a velocity distribution. Supply a measured or otherwise justified mean velocity.
Can an empty pipe have non-zero discharge?
No. Zero wetted area gives zero flow. A reverse velocity calculation requires positive wetted area.