To accurately calculate the flow rate of an existing pool a flow meter is used. Flow meters are placed after the filtration system of the pool to determine the flow rate of a particular body of water. A flow meter is required by most states on commercial pools and spas. A flow meter will enable a service professional, or health inspector, to determine whether or not the pool or spa recirculating pump is pushing an adequate amount of water to ensure the desired water turnover (filtration). Although typically used on commercial pools flow meters are becoming increasingly popular on residential pools. With a flow meter you can more accurately calculate runtime and replacement pump size.