The king mattress size dimensions are 76 inches wide by approximately 80 inches long – about 18 inches wider than a queen. This is the closest a couple can come to having as much personal space (38 inches) as each would have on a twin bed. In fact, two twin extra-long beds pushed together are about the same size, an option that can offer a flexible arrangement for a guest room. Recommendations: A king size mattress is the best choice for couples that want maximum personal sleeping space. It's also the best bet to accommodate that time on Sunday morning when children may pop into bed – or if the pets in your house have mattress privileges! However, size can be a problem. Even with a "split boxspring" – two twin-size foundations to support it – to make a king set more manageable, it may be too big for some bedrooms or too bulky to navigate through some corridors, doors and stairwells. It's a good idea to break out the tape measure to check your room dimensions if there is any doubt.