A twin bed is 39 inches wide by 75 inches long. A twin-XL bed is extra long, being the same width but five inches longer than a regular twin. Twin beds are made to accommodate one child or one adult sleeper.
The twin-XL size is better for taller youth and adults and is the same length as a queen- or king-size bed. Two twin-XL beds side by side are equal in size to a king-size mattress. Twin beds are good for bunk beds, day beds, or small guest rooms. Other common mattress sizes include full, queen, king and California king.