Weir Flow Calculator
Calculate open channel flow over a sharp crested weir from the head measured upstream, for the most common weir shapes.
- Q = 8/15 × Cd × √(2g) × tan(θ/2) × H2.5 (SI units, H and L in m)
- Q = 0.024506 m³/s
Weir Equations (SI units)
Rectangular contracted: Q = 1.84 × (L − 0.2H) × H3/2
Rectangular suppressed: Q = 1.84 × L × H3/2
Cipolletti: Q = 1.86 × L × H3/2
Q in m³/s, H is the head above the crest measured 3 to 4 H upstream, L the crest length and θ the notch angle. A 90° V notch with Cd = 0.58 gives Q ≈ 1.37 H2.5.
Worked example
A 90° V notch with 0.2 m head: Q = 1.37 × 0.22.5 = 0.0245 m³/s = 88 m³/h.
Frequently Asked Questions
Which weir is best for low flows?
The V notch, because a small flow still produces a measurable head.
Where should the head be measured?
Upstream of the weir at a distance of 3 to 4 times the maximum head, where the water surface is not yet drawing down.
Do these formulas need free discharge?
Yes. The nappe must spring clear of the weir with air beneath it; submerged weirs need correction.
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