Standard futon measurements are the same as regular mattress measurements. Standard mattress measurements for twin size are 39 inches by 75 inches, full size are 54 inches by 75 inches, queen size are 60 inches by 80 inches, and king size are 76 inches by 80 inches.
Futon mattress thickness varies between 2 and 10 inches. The average thickness is 6 inches. Thicker mattresses last longer, provide more cushion and are higher in cost. Thinner mattresses are easier to store, provide a firmer surface and are less expensive. Standard sheets fit futon mattresses, although specialty sheets and covers are also available.