The king-size mattress dimensions are 76 inches wide by approximately 80 inches long about 16 inches wider than a queen.

A standard twin bed is 39 inches wide and 75 inches long; a standard king bed is 76 inches wide and 80 inches long. Attaching two twin beds will give you a king size bed 2 inches wider than the standard size king and 5 inches short.

An Twin Bed is the smaller cousin of the the Full which is 15 inch wider but has the same length. Sometimes, for simplicity sake, the Twin is called a 'Single' and the Full is called a 'Double'.