Weir & Overflow Sizing Calculator
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Weir & Overflow Sizing Calculator
Size a rectangular or V-notch weir for a SuDS high-level overflow or spillway, or find the flow over a weir at a given head. Sharp-crested weir equations from The SuDS Manual (CIRIA C753, Chapter 28).
Rectangular weir
For overflows and spillways carrying larger flows over a level crest.
V-notch (triangular) weir
Best for low flows and accurate control/measurement — discharge is very sensitive to head.
Equations & notes
The sharp-crested weir equations and when to use each.
Rectangular weir. Q = ⅔ × Cd × √(2g) × b × H1.5. With Cd ≈ 0.62 this is the familiar Q ≈ 1.83 × b × H1.5 (SI). For a contracted weir the effective crest length is reduced by 0.1 × H per end contraction.
V-notch weir. Q = (8/15) × Cd × √(2g) × tan(θ/2) × H2.5. Flow scales with H2.5, so a small head change makes a big difference – ideal for low flows and accurate control.
| Use | Choose |
|---|---|
| High-level overflow / spillway (larger flows) | Rectangular |
| Low-flow control or measurement | V-notch (90°) |
| Very low / precise flows | V-notch (60° / 45°) |
Design notes. Keep the head over a rectangular crest below roughly one-third of the crest length for the equation to hold, and ensure free (un-submerged, ventilated) discharge. An overflow/spillway is a safety route for events beyond the design storm – provide freeboard above the design water level and protect the downstream face and channel against erosion. Confirm the structure against C753 Chapter 28 and the routing of exceedance flows.
This weir flow calculator sizes a rectangular or V-notch weir for a SuDS high-level overflow or spillway, or finds the flow over a weir at a given head. It uses the sharp-crested weir equations from The SuDS Manual (CIRIA C753, Chapter 28). High-level overflows are part of designing for exceedance under the national SuDS standards.
Rectangular weirs for larger flows
A rectangular weir carries larger flows over a level crest, which makes it the usual choice for an exceedance overflow or pond spillway. Enter the crest length and head, or ask the calculator to size the crest for a target flow. As a rule of thumb, keep the head over the crest to around a third of the crest length so the weir behaves predictably.
V-notch weirs for low flows and measurement
A V-notch (triangular) weir is best for low flows and for accurate flow measurement, because the discharge is very sensitive to head. The calculator handles the common notch angles and will either give the flow at a head or the head needed for a target flow.
Where a weir fits in a SuDS design
A weir is the safety device, not the flow control. The throttle or vortex control sets the everyday discharge; the weir passes the larger exceedance flow safely once storage is full, protecting the structure and routing water to a defined exceedance path rather than to property.
Further reading: 2025 national standards for SuDS · green SuDS: nature-based drainage · gov.uk SuDS technical standards.
Frequently asked questions
Hydrobrake is one well-known brand of vortex flow control; there are others. The device and the calculation are the same in principle, which is why this helper is written to be vendor-neutral.
Use the sharp-crested weir equation for the relevant shape, with a discharge coefficient and the head over the crest. The calculator applies the C753 equations for both rectangular and V-notch weirs.
To handle exceedance. When a storm exceeds the design event and storage fills, the weir passes the surplus safely instead of letting it back up and flood.
One of our experienced Flood Risk Consultants will get back to you within 60 minutes