Orifice and Weir Calculator
Reference page for estimating outlet-control discharge through orifices and weirs used in stormwater basins, tanks, treatment systems, and hydraulic structures.
Common outlet controls
| Outlet type | Typical use |
|---|---|
| Orifice | Controls lower-stage discharge from a basin, tank, or riser structure. |
| Rectangular weir | Controls overflow or higher-stage discharge across a straight crest. |
| V-notch weir | Measures or controls low flows with more sensitivity at shallow depths. |
| Riser structure | Combines orifices, slots, grates, weirs, and overflow protection. |
Important inputs
- Outlet geometry, diameter, width, crest length, or notch angle.
- Invert or crest elevation and water-surface elevation.
- Head on the outlet and whether the outlet is submerged.
- Discharge coefficient assumptions and tailwater conditions.
- Trash rack, clogging, maintenance, and emergency overflow considerations.
Design cautions
Outlet controls should be checked as part of a complete stage-storage-discharge analysis. Small changes in invert elevation, clogging, tailwater, or basin geometry can materially affect storage and release rates.