A king size mattress is approximately 76" wide and 80" long, but dimensions can vary depending on manufacturer and style. A California king mattress is about 72" wide and 84" long. The two are also known as Eastern and Western king mattresses, respectively.
The height of king mattresses vary among brands which accounts for differences in overall dimensions as well. The California King is the longest of mattresses, while the standard king is the same length as the queen and extra long twin mattresses. Both types of king mattresses utilize split box springs for foundations, with the standard king being equivalent to the width of a twin size mattress or frame.
California king and king size mattresses are the largest mattresses manufactured for home use. They are sometimes difficult to fit through doorways, which is why some companies produce split king mattresses. These products come in two separate pieces, each approximately the size of a single twin mattress. It is also possible to purchase split king box springs, which either come in two pieces or fold down the center.