For people in the field who have to calculate flow for flumes/weirs, open-channels, and partially full pipes, Teledyne ISCO has created a very handy application for your iOS or Android smart-phone or tablet called ISCO FlowCalc.
This app includes conversions for flumes, weirs, and area/velocity. For example, if you are working in the field and need to determine how much water is going over a weir, just tap on Weir, choose the type and size of weir and then use the slider bar to set the depth/stage to the nearest 0.01’ and the app displays your flow. You can also choose Area/Velocity, just select you channel shape (e.g. round pipe, U-shaped, trapezoidal, etc), input the dimensions, water depth and velocity and the app displays the flow in your preferred unit of measure (e.g.CFS, MGD GPM).