There are several measurement systems in use today. The English customary system is also known as the standard system, U.S. customary system (or units), or English units. It actually consists of two related systems: the U.S. customary units and the British Imperial System. The background of the units of measurement is historically rich and includes modern familiar terms, such as foot, inch, mile, and pound, as well as less well-known units, such as span, cubit, and rod. The official policy of the United States government is to designate the metric system as the preferred system for trade and commerce; but customary units are still widely used on consumer products and in industrial manufacturing.

In order to link all systems of weights and measures, both metric and non-metric, there is a network of international agreements supporting what is known as the International System (SI). It is abbreviated as SI (but not S.I.), in reference to the first two initials of its French name, Système International d’Unités. It was developed from an agreement signed in Paris on May 20, 1875 known as the Treaty of the Meter (Convention du Mètre). To date, 48 nations have signed the treaty. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, or Bureau International des Poid et Mesures). Because there is a need to change or update the precision of measurements over time, the SI is updated every few years by the international General Conference on Weights and Measures (CGPM, or Conférence Générale des Poids et Mesures), the two most recent meetings being 2007 and 2011. SI is also referred to as the metric system, which is based on the meter. The word can also be used in mathematics (for example, metric space) or even computing (fontmetric file). It is often referred to incorrectly as “metrical”. (See below for more about the metric system.)