To determine how many gallons of water would be held by a tank of specified dimensions, first find the contents in cubic feet, and then multiply by 6'23. The contents of a rectangular tank 6 ft. by 9 ft. by 4 ft. 6 in., equals 6 x 9 x 41/2 = 243 cub. ft., so that the water contained should measure 243 x 623 = 1,514 gal. (approximately).