When used for flow measurement, a weir plate is a barrier across a body of water over which water flows.  Spanning the full width of the channel or body of water, the shape of a weir plate generates a known level-to-flow relationship.  For a weir to be accurate, the approaching flow must be both subcritical and under open channel (flowing with a free surface).

In a properly sized weir pool and with the proper flow conditions, weir plates can exhibit laboratory accuracies of +/-2%, although field accuracies of +/-5% are more realistic.

The notch of a weir plate is usally a "V" shape or a rectangular cut-out, although Trapezoidal (Cipolletti), Circular, Compound, and Proportional weirs have been developed.