Metric and Customary Systems

A brainchild of France, the metric system, or decimal-based measurement protocol, has not yet been adopted fully by many countries including the United States. This results in a different measurement unit for just about everything.

The U.S. Customary System is an inch-pound system having about 300 distinct units of measurement. A football field is measured in yards, while a race goes by meters. Air pressure is measured in PSI(for tires) but surface atmospheric pressure is calculated in inches on mercury, and air pressure in millibars.