The flow of solution through these narrow channels is determined by the Pouiselle Flow equation (given by Equation 1) where the drop in pressure within a slot-die head (delp) is determined by the flow rate of the solution (V), the viscosity of the solution (mu), the channel length (L), and the channel width (b). The width of the slot opening determines the flow rate. To calculate the flow rate of a Slot Drain, you can use these formulas: 1/2 inch slot opening = 11 gallons per minute per foot of slot 1 inch slot opening = 18 gallons per minute per foot of slot. A slot is a weir regardless of whether it fits into any standard model of a weir. The important characteristic of a weir that it is free surface flow and at some point the flow is at minimum energy (at transition from sub critical to super critical critical depth if you like). You can calculate the flow through a weir/slot from this principle.