Orifice and Weir Calculator
Estimate discharge through a circular orifice or over a simple rectangular weir.
Orifice and Weir Calculator
Estimate discharge through a circular orifice or over a simple rectangular weir.
Method notes
The orifice option uses Q = Cd A sqrt(2gH). The rectangular weir option uses Q = Cd (2/3) b sqrt(2g) H^(3/2). Confirm local design criteria and coefficient assumptions before design use.
This is a simplified hydraulic screening tool.
What this calculator does
This tool provides simplified discharge estimates for orifice and rectangular weir conditions using head, opening size, and discharge coefficient.
Inputs explained
- Method: Select orifice or rectangular weir.
- Diameter or width: Orifice diameter or rectangular weir width.
- Head: Driving water head above the orifice centerline or weir crest.
- Discharge coefficient: Coefficient used to account for contraction and losses.
Limitations
This simplified tool does not account for submerged flow, tailwater, approach velocity, complex outlet structures, trash racks, clogging, or detailed stage-discharge curves.
FAQ
What head should I use for an orifice?
For a small orifice, head is commonly measured from the water surface to the centerline of the opening.
What head should I use for a weir?
For a rectangular weir, head is measured above the weir crest.
What discharge coefficient should I use?
Use the coefficient required by the local standard, hydraulic reference, or manufacturer. The default is only a starting point.
Can this size a detention outlet?
It can support early checks, but final outlet sizing should use a stage-discharge relationship and routing.
Related tools
Need help reviewing this result?
Use this calculator for planning-level checks, then request help if you need a second look at assumptions, units, stormwater criteria, pump sizing, storage volume, or report-ready calculations.