A double bed, also known as a full-sized mattress, is 54 inches wide by 75 inches long. Some mattresses in the double bed range may be slightly smaller than the given dimensions in order to fit into the bed frame.
A double bed mattress is the next standard size up from a twin bed or single mattress. The mattress is designed to fit two small adults or one large adult. They are somewhat smaller than a queen-sized mattress and give about 27 inches of space to each bed occupant if used by two people. Double beds used to be the standard mattress size for couples, but as of 2015, queen-sized mattresses have replaced them as the mattress size of choice.