Shelf depth is determined by the size of the items to be stored, generally within the range of 6 to 24 inches. Shelf depth for a general purpose bookshelf is usually 10 to 12 inches. Shelf spacing will generally range from 7 to 15 inches with 8 to 12 inches being common for bookshelves.Click to see full answer. Similarly, how wide is the average bookshelf? 31 inches Subsequently, question is, how do you measure a bookshelf? How to Measure for Bookshelves Clear any objects from the wall and floor. Set these objects aside. Place a tape measure on one end of the wall. Pull the measure to the other end of the wall to measure the length of the wall. Pull the tape measure from the bottom of the wall to the top of the wall to measure the height of the wall. Also to know is, how wide is a Billy bookcase? IKEA Billy Bookcases come in height variations from tall at 93 1/4” (237 cm) to short at 41 3/4” (106 cm). Widths range from wide designs at 31 1/2” (80 cm) to thin designs at 15 3/4” (40 cm). All bookcases have a depth of 11” (28 cm).How deep should a shelf be? 16 to 20 inches