How much fall would you need at 1/8 of an inch per foot fall in a 90 foot run?? 12.25 inches per foot fall b. 10.50 inches per foot fall c. 11.30 inches per foot fall d. 11.25 inches per foot fall Drop = Pitch x Run Drop = 1/8 x 90 Drop = .125 x 90 Drop = 11.25 inches per foot fall