A standard twin-sized mattress measures 39 inches in width by 75 inches in length. Some manufacturers list standard twin-sized mattresses at 38 inches in width. Twin-sized mattresses, sometimes called single mattresses, sleep one person and are popular in children’s bedrooms.
An extended twin-sized mattress, often called a long twin mattress, measures 39 inches wide by 80 inches in length. There are also 38-inch-wide versions. College dorm rooms often come equipped with extended-length twin-sized mattresses.
The depths of twin-sized mattresses vary widely among manufacturers. Most mattresses are 9 to 10 inches in depth. However, pillow-top mattresses may be as deep as 16 inches.