Capacity (Q) is normally expressed in gallons per minute (gpm). Since liquids are essentially incompressible, there is a direct relationship between the capacity in a pipe and the velocity of flow. This relationship is as follows:
Where
A = area of pipe or conduit in square feet.
V = velocity of flow in feet per second.
Q = Capacity in gallons per minute

NOTE: On vertical pumps the correction should be made to the eye of the suction or lowest impeller.