Weir & Orifice Flow Calculator
Outlet Control Discharge — US Units
Compute discharge through outlet control structures: rectangular and V-notch weirs (Q = Cw·L·H^(3/2)) and sharp-edged orifices (Q = Cd·A·√(2gH)).
WWeir
Typical: ~3.33 rectangular sharp-crested, ~2.49 for a 90° V-notch (US units).
Weir Discharge
4.71
CFS
Weir Discharge
2114
GPM
OOrifice
~0.60 sharp-edged, ~0.80 short tube, ~0.90–0.98 rounded / bell-mouth.
Area = πD²/4 = 0.0873 ft².
Water surface to the center of the opening.
Orifice Discharge
0.59
CFS
Orifice Discharge
267
GPM
About These Equations
Weirs control the upper (high-flow) stages of a detention outlet; orifices restrict low flows to meet small allowable release rates. Many outlet structures stack a low orifice under a higher weir or overflow to control several design storms at once.
Q = Cw L H^(3/2)
Rectangular weir · Cw ≈ 3.33
Q = Cw tan(θ/2) H^(5/2)
V-notch weir · Cw ≈ 2.49
Q = Cd A √(2gH)
Orifice · Cd ≈ 0.60 · g = 32.2 ft/s²
Related Calculators & Products
Design the Whole System
- Detention Volume Calculator — size storage; this tool sizes the outlet that sets the release rate.
- Runoff Calculator — establish the peak inflow your outlet must meter down.
- Pipe Sizing Calculator — size the outlet pipe downstream of the control structure.
- Open Channel Flow Calculator and the flow regimes guide.
Build the Structure
- Catch basins often house the outlet control weir or orifice plate.
- Detention systems upstream of the controlled outlet.
- Flow control valves and NDS valves for active release control.
- Drainage grates for inlet protection at the structure.