King size mattress dimensions have a width of 5 foot and a length of 6 feet 6 inches, or 150cm wide and 200cm long. So, they are the same length as a super king, but 30cm narrower. This makes king size mattresses ideal for those that need extra leg space but can't fit a super king in their bedroom. Browse our wide range of double mattresses in memory foam, pocket spring and more by leading UK brands. If you require any information, please feel free to contact is on 01282 614323.