The sizes of containers depend mainly on their external dimensions, so that, for example, a container can be an ISO standard "Series 1 Freight container, Rating IAA" with external dimensions of 40 feet (length) x 8 feet (width) x 8feet 6 inches (height). The dimensions are used in either imperial or metric units. Although much of the world has become metricated, the "box" or container is still referred to by its imperial units, for example, a FEU is a forty-foot equivalent unit (of space occupied).