Common Unit Systems

Introduction to Unit Systems

A unit system, or system of measurement, is a system comprised of interrelated units of measurement. Various unit systems have existed throughout history, and their importance remains evident today, as seen by their widespread use within society.

In the past, unit systems were defined locally, and often, highly arbitrarily. As such, the length of a unit could vary significantly from region to region. For example, since some units in the past were often based on parts of the body, the unit of the "foot" could have a different definition based on the size of the foot of the king or feudal lord of a given region.

With the advent of globalization, particularly the growth of commerce and science, the arguable need for a universal system of measurement became more apparent. The three common unit systems that are in use today are the International System of Units, United States customary units, and the imperial system of units.

International System of Units (SI)

The International System of Units (SI) is the modern form of the metric system and is comprised of seven base units that use twenty metric prefixes to denote decimal multiples or submultiples of the base unit. SI is intended as a coherent, rational system of measurement.

United States Customary Units

United States customary units (UCS) are a system of measurements used in the United States. UCS originated from English units and is primarily used for commercial, social, and personal applications, while the US uses the International System of Units (SI) in science, medicine, industry, government, and military.

Imperial System

The imperial system of measurements, also known as British Imperial, was defined in 1824, replacing the previous English units. Although the United Kingdom has technically adopted SI, in practice, the UK still commonly uses imperial units in everyday life.

Common Conversion Examples