The Friday File: An NBA basketball’s diameter is between 9.43 and 9.51 inches. The hoop has a diameter of 18 inches. A ball hitting the center has roughly 4.25 inches between the edge of the basketball and the hoop. Shots three inches off center go in 74% of the time. If off four inches the percentage falls to 67%. It falls to 55% at five inches and 28% at six.