For an approximation of the output of a vertical pipe, use the following formula:

GPM = H x K x D2 x 5.68
GPM	=	gallons per minute
H	=	height in inches
D	=	diameter of pipe in inches
K	=	constant from 0.87 to 0.97 for diameters of 2 to 6 inches and heights (H) up to 24 inches
eg. K = 0.97, 6 inch diameter with 12 inch height =~ 687 gpm