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Common 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. Since travel was more limited in the past, local definition and use of units could be seen as being more practical. However, with the advent of globalization, particularly the growth of commerce and science, the arguable need for a universal system of measurement became more apparent.

Standardized units of measurement facilitate communication between different cultures and countries that may otherwise choose to use local systems, potentially leading to confusion and miscommunication. The International System of Units (SI), the most widely used system of measurement today, was developed in an effort to provide a standardized, more rational system that could be used worldwide. Despite efforts to implement SI globally, there are still a few unit systems in common use, including the United States customary units, and the imperial system of measurement (though most countries that have not officially adopted SI still use SI to a certain extent).

Although striving for standardization is important, since it is difficult to entirely eliminate the local use of historical unit systems, it is equally important to accept that other systems of measurement exist and be able to use them, or at least relate and convert them to the preferred unit system.

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

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. It is a system that was stringently conceived and is defined based on invariant constants of nature including the speed of light, the triple point of water, and a physical prototype. In order to maintain reliability and precision, SI requires the precise definition of constants in addition to precise of measurement standards. As such, it is an evolving system that changes when more stable constants are discovered, or other constants can be more precisely measured. SI is the most widely used system of measurement, and the evolution of the system is still ongoing today.

The SI base units as well as a table of metric prefixes (courtesy of Wikipedia.org) are listed below:

SI base units:

Metric prefixes:

Metric prefixes in everyday use
micro μ0.00000110-6

SI-derived units:

In addition, SI also includes 22 units of measurement that are derived from the 7 SI base units. These units are either dimensionless or are expressed as a product of one or more of the SI base units. Some of the more common examples of these include:

Non-SI units accepted for use with SI:

There are also numerous units that are accepted for use with SI that are not considered SI units or SI derived units. Some of the more common examples include:

History of SI:

The International System of Units (SI) is the most widely used unit system in the world. It is the modern version of the metric system which was developed over time while borrowing units or ideas from other systems, in large part by a committee of the French Academy of Sciences, beginning in 1791.

The process of metrication, including the eventual definition and adoption of the International System of Units was a slow process. France officially introduced the metric system in 1799, and the system spread throughout Europe during the 19th century. By the 1970s, metrication in the form of SI was complete in almost all countries around the world. Notable exceptions to this include the United Kingdom, the United States, Liberia, and Myanmar. Excluding the United Kingdom, these countries are the only countries that have not officially adopted SI.

The United States:

The United Kingdom:

Although there are other countries in which units other than SI are primarily used, the US and the UK are the largest contributors to two of the other unit systems commonly used today. As such, the other countries, many of which use unit systems influenced by either the UK or the US, will not be discussed here.

United States customary units

United States customary units (UCS) are a system of measurements used in the United States (US). UCS originated from English units (not to be confused with imperial units) which were used by the British Empire starting as early as 1495 and continued to be used in the UK up until replacement by the imperial system in 1824. While UCS is primarily used for commercial, social, and personal applications, the US uses the International System of Units (SI) in many other areas including science, medicine, industry, the government, and the military.


Units of length:

The four US customary units that are in everyday use are listed below, along with their SI equivalents based on the definition of 1 yard as 0.9144 meters:

Units of area:

Square feet, square inches, square yards, etc., are units of area commonly used in UCS, but the only area measurement in the system that is not related to one of its lengths is the acre, which is equal to 4,046.873 m2.

Units of volume:

UCS uses many different measurements of volume. It uses some general units of volume, as well as volume measurements specifically for dry or liquid volumes. Below are some, but not all of these volumes, and their SI equivalents.

General approximate units of volume:

Approximate liquid volumes:

These volumes have similar names as their counterparts in the imperial system, but the actual measurements differ slightly. Also, for most of these measurements, the US simply adds the term "dry" before the unit to distinguish a unit from its liquid definition. This distinction does not exist in the imperial system, which does not have separate dry or liquid volumes.

Approximate dry volumes:

Units of weight and mass:

The most commonly used system of mass in the United States is avoirdupois weight. Troy weight is sometimes used, but not widely. UCS is based on the avoirdupois pound, which was defined in 1959 as exactly 453.59237 grams. The pound is sometimes used as a force, using the term "pound-force." This should not be confused as a unit of mass. Below are listed some of the USC units of mass, along with their approximate SI equivalents:

Units of temperature:

UCS uses the unit degrees Fahrenheit to measure temperatures for everyday purposes. Like most of the rest of the world however, UCS uses degrees Celsius and kelvins within scientific contexts.

Imperial system

The imperial system of measurements, also known as British Imperial, was defined in 1824, replacing the previous English units known as the Winchester Standards, which were in effect from 1588 to 1825. Although the United Kingdom has technically adopted the International System of Units (SI), in practice, the UK is much like the United States in that it uses SI in many areas such as industry, healthcare, government, science, and others, but still commonly uses a system derived from English units (the imperial system) in everyday life. The imperial system is also still in use in Canada as well as some other countries previous under the control of the British Empire.

Below are some of the various units of the imperial system, and their approximate metric equivalents. Many of these are similar to units in the US customary system.

Units of length:

Units of area:

Units of volume:

While these units have the same names in US customary units, their values are different, and the imperial system does not have separate dry or liquid volumes.

Units of weight and mass:

These units are similar to their UCS counterparts with the exception of the ton. The imperial ton (2,240 pounds), commonly referred to as the long ton in the US, is much closer to the metric ton (2,204.6 pounds) and is larger than the US short ton (2,000 pounds).

While the stone is related to the ounce and the pound, and would have the same measurement in USC, the stone is not used in the United States, and is mainly widely used as a measurement of body weight in the UK.