Unit Prefix - Metric Prefixes

Metric Prefixes

The prefixes of the metric system precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol. Many of the prefixes date back to the introduction of the metric system in the 1790s others are more modern. There are currently twenty metric prefixes standardised by the International Bureau of Weights and Measures in resolutions dating from 1960 to 1991.

Although formerly used, the SI disallows combining prefixes; the microkilogram or centimillimetre, for example, are not permitted. Prefixes corresponding to powers of one thousand are often recommended, however, such units as the hectopascal, hectare, decibel, centimetre and centilitre are commonly used. When units occur in exponentiation prefixes are considered part of the unit; 1 km2, for example, means one square kilometre not one thousand square metres and 1 cm3 means one cubic centimetre not one hundredth of a cubic metre.

In general prefixes are used with any metric unit and may even be used with non-metric units, however, some combinations are more common than others. The choice of prefixes with a given unit is usually dictated by convenience of use, unit prefixes that are much larger or smaller than encountered in practice, are seldom used, albeit valid combinations. In most contexts only a few, the most common, standard combination are established. Prefixes for multiples greater than one thousand, for example, are rarely applied to the gram or metre.

Some of the prefixes used in older versions of the metric system but are no longer used. The prefixes myria, demi- and double-, denoting a factors of 10000 (from the Greek μύριοι, mýrioi), 1⁄₂ and 2 respectively, were parts of the original metric system adopted in France in 1795 but they were not retained when the SI prefixes were agreed internationally by the 11th CGPM conference in 1960. The prefix "myria-" has also been spelt "myrio-".