# Unit of measurement

A unit of measurement is the magnitude of a quantity as defined and adopted by convention or law. Any quantity of the same kind can be expressed as a multiple of a given unit of measurement.

Centimeters (length), pound (weight), acre (area), liters (volume), and seconds (time) are examples of commonly used units of measurement.

## Standardization of units of measurement

Many different systems of measurement have been used throughout history. Even today, different regions of the world may use different systems of measurement locally. For example, the United States largely still uses the U.S. customary system for everyday applications, while the United Kingdom uses the imperial system. Both also use the International System of Units (SI).

SI is the modern form of the metric system and is the most widely used system of measurement around the world. It is currently the only system of measurement that has an official status in virtually every country in the world. It was developed largely due to the need for standardization.

As a global society, it is necessary to be able to communicate effectively with other regions. Having too many different systems of measurement, and needing to convert between them when working with other countries can, and has, lead to mistakes with serious consequences. One such example was the NASA Mars Climate Orbiter, which failed due to the miscommunication of units of force (newton vs pound force).

Thus, while many countries around the world still use various units of measurement locally, for scientific, business, military, and many other purposes, SI is used to maintain a clear standard. This generally eases communication between countries and assists in preventing mistakes.

Just as an example, below are a few commonly used units of length:

- mile
- fathom
- foot
- yard
- league
- chain
- rod
- light year
- parsec

There are many more, but in order to relate each value, which vary significantly in magnitude (and not by some pattern or constant measure), it is necessary to know what conversion factor to use. In contrast, SI uses base units and prefixes which denote a power of 10 by which the base unit is multiplied. Thus, all measurements of length use the same base unit, the meter (m) along with prefixes to allow for varying magnitudes that are all relatable by a given power of 10.

- millimeter = 10
^{-3}m - centimeter = 10
^{-2}m - decimeter = 10
^{-1}m - meter = 10
^{0}m - kilometer = 10
^{3}m - yottameter = 10
^{24}m

All the SI prefixes are applicable to each SI base unit of measurement. This, along with the use of SI derived units provides a coherent system of units of measurement that serves as a standard worldwide.