Weir Flow Calculator
Calculate flow rate over a rectangular sharp-crested weir from its crest length and the head (depth) of water flowing over it.
How this calculator works
A weir is a low wall or notch that water spills over, and the depth of water flowing over that crest tells you how much flow is passing. This calculator uses the standard formula for a rectangular sharp-crested weir, taking the crest length and the head — the depth of water flowing over it — to estimate flow rate.
Formula: Q = 3.33 × L × H^1.5, where L is the weir’s crest length in feet and H is the head in feet.
Worked example
A 3 ft weir crest length with a 0.5 ft head:
- Crest length (L): 3 ft
- Head (H): 0.5 ft
- Flow: Q = 3.33 × 3 × 0.5^1.5 ≈ 3.53 cfs
Notes
A weir is a flow-measurement structure — a notch or low dam that water flows over. This rectangular formula is one of several standard weir equations used in hydraulics. A V-notch weir, which uses a triangular notch rather than a rectangular one, follows a different formula, since its flow area changes shape as the head rises instead of staying a constant width. V-notch weirs are common where smaller flows need to be measured precisely.
This rectangular version is the standard used for spillways, flow gauging stations, and pond overflow structures, where a flat, wide crest makes for a simple and reliable measuring point.
How to use
Enter the weir’s crest length and the head — the depth of water flowing over the crest, measured upstream of the crest — both in feet. The calculator returns the estimated flow rate in cubic feet per second using the standard rectangular sharp-crested weir formula.
A planning estimate using the standard rectangular weir formula. Precise flow measurement applications should confirm coefficient values for the specific weir geometry with a hydraulics reference or engineer.
Frequently asked questions
What is a weir and why does it measure flow?
A weir is a notch or low dam that water flows over, built into a channel, spillway, or pond outlet. Because the depth of water flowing over the crest relates predictably to how much flow is passing, measuring that depth lets you calculate the flow rate without needing more complex instruments.
How do I calculate flow over a rectangular weir?
Use Q = 3.33 x L x H^1.5, where L is the weir's crest length in feet and H is the head — the depth of water flowing over the crest — in feet. A 3 ft weir with a 0.5 ft head gives Q = 3.33 x 3 x 0.5^1.5 ≈ 3.53 cubic feet per second.
Is the rectangular weir formula the same for all weir shapes?
No. This formula applies to a rectangular sharp-crested weir. A V-notch weir, which uses a triangular notch instead of a rectangular one, follows a different formula because its flow area changes shape as the head rises rather than staying a constant width. V-notch weirs are common for measuring smaller flows precisely.
Where is the rectangular weir formula typically used?
The rectangular sharp-crested weir formula is the standard used for spillways, flow gauging stations, and pond or detention basin overflow structures, where a wide, flat crest is a practical way to measure or control flow.
Estimates only. Verify quantities with your supplier before purchasing.