This calculator estimates discharge over a circular weir installed at the outlet of a partially full pipe. It uses the published “General Circular Weir Flow” relationship attributed to Addison (1941) and reproduced in Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance. Inputs can be entered in centimeters or inches and are converted internally to decimeters as required by the original equation. A practical installation check is included: the pipe inside diameter should exceed the weir opening diameter by at least 2 in.
Circular Weirs for Measuring Flows in Partially Full Pipes Calculator
January 23, 2026
Circular Weirs for Measuring Flows in Partially Full Pipes Calculator
Input (Metric + Imperial linked)
Definition sketch
Results
h/d
0.4115
Cd
0.59
Q (Metric), L/s
43.974
Q (Imperial), cfs
1.553
Equations
Discharge coefficient
Cd = 0.555 + 1110 · (h/d) + 0.041 · (h/d)
where h/d is dimensionless.
Flow equation
Q = Cd · [ 10.12 · (h/d)1.975 − 2.66 · (h/d)3.78 ] · d5/2
Q in L/s, and d, h in dm (this tool converts from cm/in to dm internally).
How to citeShowHide
Author ORCID: 0009-0000-1316-1362
APA
Chernitsky, I. (2026). Circular Weirs for Measuring Flows in Partially Full Pipes Calculator [Computer software]. Chernitsky.net. https://chernitsky.net/blog/Circular-Weir-Calculator
IEEE
I. Chernitsky, "Circular Weirs for Measuring Flows in Partially Full Pipes Calculator," Chernitsky.net, 2026. [Online]. Available: https://chernitsky.net/blog/Circular-Weir-Calculator
BibTeX
@misc{chernitsky2026circular,
author = {Chernitsky, Ilia},
title = {Circular Weirs for Measuring Flows in Partially Full Pipes Calculator},
year = {2026},
howpublished = {\url{https://chernitsky.net/blog/Circular-Weir-Calculator}},
}