Weir flow from the head over the crest
Choose the weir type, enter the head H and the crest width B in feet, and the flow appears in cfs, gpm and million gallons per day (MGD) at the same time, each with the SI value in parentheses. A suppressed rectangular weir uses Q = 1.84 x B x H raised to 1.5, a contracted rectangular weir uses the width less 0.2 times the head, and a 90 degree V-notch weir uses Q = 1.4 x H raised to 2.5, all with Q in m³/s and B and H in meters.
These coefficients come from secondary hydraulics material and are approximations. A real weir has a discharge coefficient that moves with the head and the geometry, and the head to crest height limit quoted as an application range differs between sources, so it has not been confirmed in an authoritative source. Because these coefficients are SI values, the page converts your feet to meters, applies the formula and converts the flow back; in US units the same relations work out to about Q = 3.33 x B x H^1.5 and Q = 2.54 x H^2.5 with Q in cfs and B and H in feet. Figures are current as of October 2026.
Channel section design, approach velocity correction and the location of the head measurement are not covered. This calculator is for reference and does not replace design work; review by a qualified engineer is required.
Frequently Asked Questions
The flow contracts at both ends, so the effective width is smaller than the crest. This page subtracts 0.2 times the head from the width you enter.
No. The 90 degree V-notch equation uses the head only, so anything entered as a width is left out of the calculation.
No. Sources quote different coefficients, and picking one freely would widen the error, so only the confirmed values are used.