Temporary flow measurement during startup and testing: sharp-crested V-notch, rectangular, and Cipolletti weirs, plus submerged orifice. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: These formulas assume a properly built and installed sharp-crested weir with a free (aerated) nappe, or a clean sharp-edged orifice, and head measured correctly. A plywood weir with a rough edge, a drowned nappe, or a tape measure held at the crest reads confidently and wrong. Good field accuracy is ±5% under ideal conditions; treat results accordingly.
Results
Flow
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Formula used
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How the Numbers Are Derived
Weir formulas (US units: Q in cfs, h and L in ft)
90° V-notch (USBR): Q = 2.49 · h^2.48
V-notch, other angles: Q = 2.5 · tan(θ/2) · h^2.5 (standard field form)
Cipolletti: Q = 3.367 · L · h^1.5 (the 1:4 side slopes compensate for end contractions)
Submerged orifice: Q = C_d · A · √(2gΔh)
Conversions: 1 cfs = 448.83 GPM = 0.6463 MGD.
Conditions the formulas assume (the accuracy fine print)
Head range: most sharp-crested weir formulas are rated for roughly 0.2–2.0 ft of head. Below ~0.2 ft, surface tension and nappe clinging dominate and accuracy falls apart — the tool warns.
Free nappe: air must circulate under the falling sheet of water. A nappe that clings to the downstream face over-reads. Suppressed rectangular weirs need vents.
Approach conditions: low approach velocity (weir pool, not a flume), crest well above the downstream water surface (no submergence), and the crest at least ~2× the head above the upstream channel bottom.
Contracted weirs: the Francis end-contraction correction assumes the notch is well away from the channel sides (≥ 2h each side).
Sharp crest: the upstream edge should be a true sharp edge (¼-in. plate chamfered downstream, or angle iron) — a 2× lumber crest is a broad-crested weir with a different, mushier coefficient.
Choosing a device in the field
V-notch for low/variable flows (best sensitivity at low Q — a 90° notch reads roughly 0.9 to 450 GPM over 1–12 in. of head).
Rectangular or Cipolletti for higher flows where a V-notch would need excessive head.
Submerged orifice where there isn't enough drop for a weir to run free.
For anything that needs defensible accuracy over days (billing, permit reporting), rent a flume or a proper meter — a temporary weir is a startup verification device.
References
USBR, Water Measurement Manual — weir and orifice formulas, installation conditions. https://www.usbr.gov/tsc/techreferences/mands/wmm/