Standard bed sizes are based on standard mattress sizes, which vary from country to country. Bed sizes also vary according to the size and degree of ornamentation of the bed frame. Dimensions and names vary considerably around the world, with most countries having their own standards and terminology. In addition, two mattresses with the same nominal size may have slightly different dimensions, due to manufacturing tolerances, amount of padding, and support type. Mattress sizes may differ from bedding sizes.

A mattress is a large, usually rectangular pad for supporting a lying person. It is designed to be used as a bed, or on a bed frame as part of a bed. Mattresses may consist of a quilted or similarly fastened case, usually of heavy cloth, containing materials such as hair, straw, cotton, foam rubber, or a framework of metal springs. Mattresses may also be filled with air or water.