The size of a regular queen size box spring is 60 x 80 inches. A split queen box spring is 30 x 80 inches, which is half the width of a regular foundation. A regular king size box spring is 76 x 80 inches. The split king box foundation is half the width, measuring 38 x 80 inches. Regular California king box springs are 72 x 84 inches, but a split California king foundation is 36 x 80 inches.
Split box springs are placed side by side in the bed rail to make the bed size 78 inches wide for a king size bed, 60 inches for a queen and 72 inches for a California king size bed. The mattress is placed on top of the split foundation. In some instances, you may need a split mattress for the split foundation if you have tight corners or low ceilings in a stairway.
Split queen, king and California king size mattresses are the same dimensions of the split box spring. The benefit of using a split mattress is that you don't need to bend, twist or squeeze the mattress through tight areas to get it into a bedroom. The split mattress is placed on top of the split foundation.
A few manufacturers make sheet sets for split mattresses, or you can use a set of bed sheets that would fit a regular king or queen size mattress. The split mattress sheet set includes two fitted sheets, one flat sheet and two pillowcases. If you don't want to separate the mattresses with a fitted sheet, a regular sheet set with one fitted sheet will cover the entire mattress because the combined size of the two split mattresses is the same size as a one-piece mattress.