There are two types of king-sized beds, the Standard King and the California King, and the two sizes differ from each other by about four inches in each direction. It is important to pay close attention to the differences between the two beds, especially when buying sheets and other linens.
King-size beds are the only size to offer two sleepers each almost the same amount of room as one twin bed.
The Standard King is used in softside waterbeds, innerspring, and foam beds. Standard size of the mattress is 78" X 80" but quite often manufacturers are a little off in their sizes. 76" X 80" is also a popular standard King size.
The California King is also the standard for Waterbed King since waterbeds were invented in California. This is 72" X 84" Why is California King much taller? With wood frame waterbeds it's important that it be taller so that 6 foot sleepers are not scrunched up.
When you buy linens be sure they are deep enough. Usually bargain brand sheets are made for a 6 inch mattress and this simply won't do on modern deep mattresses. Wood frame waterbeds are about 9 1/2" deep and many softsides are much deeper.