The American system of measurement is known as the U.S. customary system. Most of the units of measurement in the customary system are derived from British imperial units of measurement.
Because the American system of measurement is very similar to the British imperial system, units are also commonly referred to as imperial units. The units have many different origins. The pound, for example, comes from the Roman Empire, while the inch has origins in Anglo-Saxon England and is thought to represent the length of three barley corns. The only other countries in the world that use these measurements are the United Kingdom, Canada, Australia and Ireland. The standard measurement system in other countries is called the metric system.