Important factors to consider when sizing a water pump: GPM and PSI Ratings Inlet and Outlet Size for Hoses Vertical and Horizontal Distance you need to Pump Fluids Tips on How to Choose the Right Inlet/Outlet Size If the pump you need has different inlet/outlet size options, all you need to know is that they all work the same way. The water pump sucks in water through an inlet valve and ejects it from a discharge valve. A larger valve can potentially finish a job much faster than a 1-inch pump.