SI prefix
Sign in to savestandardized prefix of the SI (Système International) for measurement units, indicating a multiple or fraction as a power of 10 or 1000
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SI prefixes are standardized abbreviations added to measurement units that represent powers of 10 or 1000, making it easier to express very large or very small quantities—for example, "kilo" means 1,000 and "milli" means 1/1,000. They matter because they provide a consistent, universal way to scale measurements across science, medicine, and engineering, so everyone using the metric system can communicate precisely without writing out long strings of zeros.
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A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decimal. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix kilo, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli, likewise, may be added to metre to indicate division by one thousand, so one millimetre is equal to one thousandth of a metre.
Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s. 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. Since 2009, they have formed part of the ISO/IEC 80000 standard. They are also used in the Unified Code for Units of Measure (UCUM).