This Demonstration calculates the water flow through an open channel. A rectangular weir is the most commonly used thin plate weir. You can vary the weir width as well as water flow height. The discharge effect, viscosity effects, and surface tension are not considered.

Flow is calculated as , where is the height of the water flow above the weir, is the velocity, and is the differential volume of water flow through the weir of width . This gives as , which implies , with use of the formula .