Folded into halves or thirds as part of convertible furniture, futon mattresses can appear smaller than standard beds. They do, however, come in the same standard sizes as most beds, including twin, full and queen. Although queen-size futon mattresses are slightly longer, the width of the futon frame can tell you what size mattress is needed.
Twin futons are not as common as full and queen but are available as a standard size. They are usually sold as convertible chairs. During the day the futon can be folded into a chair that seats one person and at night unfolded into a bed for one person. A twin futon mattress is 39 inches wide by 75 inches long, the same size as a standard twin spring or foam mattress.
Full-size futon mattresses are more common than twin but not as popular as queen-size mattresses. Convertible love seat futons are usually the same size as a standard full bed when laid out flat. They seat two people sitting close together, and two small people can sleep on the futon when it is extended. A full-size futon mattress measures 54-by-75 inches, the same as a full-size mattress.
Most futons that are folded in half for seating like a couch have queen-size mattresses. Tri-fold futons, in which a third of the mattress drapes over the back of the frame when folded, are also popular in queen size. A queen-size futon mattress, measuring 60-by-80 inches, is wide enough for three people to sit closely together or two people to sleep when the frame is laid out flat. Like twin and full size, the futon mattress for a queen-size frame is the same size as a standard spring or foam bed mattress.
When measuring a futon frame to determine the mattress size, measure only the interior where the mattress will sit. Do not include arms, rails or decorative edges in the measurement. To measure the width, which determines the mattress size needed, pull a measuring tape straight across the frame, starting at the inside of the arm or rail and ending at the inside of the opposite arm or rail. Match the measurement to standard mattress sizes to determine whether the futon is a twin, full or queen.