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Metric prefixes have also been used with some non-metric units. The SI prefixes are metric prefixes that were standardised for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 2022. [1] [2] Since 2009, they have formed part of the ISO/IEC 80000 standard.
A base unit is a unit adopted for expressing a base quantity. A derived unit is used for expressing any other quantity, and is a product of powers of base units. For example, in the modern metric system, length has the unit metre and time has the unit second, and speed has the derived unit metre per second.
Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. According to Schadow and McDonald, [ 1 ] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers.
The prefixes are never combined, so for example a millionth of a metre is a micrometre, not a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a milligram, not a microkilogram. [5]: 122 [11]: 14 The BIPM specifies 24 prefixes for the International System of Units (SI):
Prefix name N/A deca hecto kilo mega giga tera peta exa zetta yotta ronna quetta; Prefix symbol da h k M G T P E Z Y R Q Factor 10 0: 10 1: 10 2: 10 3: 10 6: 10 9: 10 12: 10 15: 10 18: 10 21: 10 24: 10 27: 10 30
The SI system after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants of nature and artefacts used to define them (such as the mass of the IPK for the kilogram).
Prefix Base 10 Decimal Adoption [nb 1]Name Symbol quetta: Q: 10 30: 1 000 000 000 000 000 000 000 000 000 000: 2022 [1]: ronna: R: 10 27: 1 000 000 000 000 000 000 000 000 000: yotta: Y: 10 24: 1 000 000 000 000 000 000 000 000 ...
The base unit in the International System of Units (SI) is the meter, defined as "the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299792458 seconds." [9] It is approximately equal to 1.0936 yd.