Water Flow (GPM/GPH) based on Pipe Size and Inside/Outside Diameters Assume "High Pressure" PEAK flow. About 18 f/s flow velocity 1-1/2"1.5 - 1.6"126 2"1.95 - 2.05"200 2-1/2"2.35 - 2.45"300 3"2.9 - 3.05"425 9 more rows