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).
List of SI prefixes
The BIPM specifies twenty-four prefixes for the International System of Units (SI).
The first uses of prefixes in SI date back to the definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use, and were recognised by the 1947 IUPAC 14th International Conference of Chemistry before being officially adopted for the first time in 1960. The prefixes that were most recently adopted are ronna, quetta, ronto, and quecto. These prefixes were adopted in 2022, after a proposal from British metrologist Richard J. C. Brown. Before 2022, Q/q and R/r were the only Latin letters available for abbreviations, with all other Latin letters either being: already used for other prefixes (a, c, d, E, f, G, h, k, M, m, n, P, p, T, Y, y, Z, z); already used for SI units (including: SI base units, SI derived units, non-SI units mentioned in the SI; A, B, C, d, F, g, H, h, J, K, L, m, N, S, s, T, t, u, V, W); easily confused with mathematical symbols (I and l are easily confused with 1, O and o are easily confused with 0, X and x are easily confused with ×); or a combination (some symbols are used for both, e.g. metre and milli- both use m). The large prefixes ronna and quetta were adopted in anticipation of needs for use in data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were also added, even without such a driver, in order to maintain symmetry. The prefixes from peta to quetta are based on the Ancient Greek or Ancient Latin numbers from 5 to 10, referring to the 5th through 10th powers of 103. The initial letter h has been removed from some of these stems and the initial letters z, y, r, and q have been added, ascending in reverse alphabetical order, to avoid confusion with other metric prefixes.
Rules The symbols for the units of measure are combined with the symbols for each prefix name. The SI symbols for kilometre, kilogram, and kilowatt, for instance, are km, kg, and kW, respectively. (The symbol for kilo is k.) Except for the early prefixes of kilo, hecto, and deca, the symbols for the prefixes for multiples are uppercase letters, and those for the prefixes for submultiples are lowercase letters. All of the metric prefix symbols are made from upper- and lower-case Latin letters except for the symbol for micro, which is uniquely a Greek letter mu (μ). The prefix symbols are always prepended to the symbol for the unit without any intervening space or punctuation. This distinguishes a prefixed unit symbol from the product of unit symbols, for which a space or mid-height dot as separator is required. So, for instance, while 'ms' means millisecond, 'm s' or 'm·s' means metre-second. Prefixes corresponding to an integer power of one thousand are generally preferred; the prefixes corresponding to tens (deci-, deca-) and hundreds (centi-, hecto-) are less common and are disfavoured in certain fields. Hence, 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes deci- and centi-, and less frequently hecto and deca, are generally used for informal purposes; the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion". These prefixes are also commonly used to create metric units corresponding to older conventional units, for example hectares and hectopascals. Prefixes may not be used in combination on a single symbol. This includes the case of the base unit kilogram, which already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg). During mathematical operations, prefixes are treated as multiplicative factors. For example, 5 km is treated as 5000 m, which allows all quantities based on the same unit to be factored together even if they have different prefixes. A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, 1 km2 denotes 1 km × 1 km = 106 m2, not 103 m2.
Usage
Examples The mass of an electron is about 1 rg (rontogram). The mass of 1 litre of water is about 1 kg (kilogram). The mass of the Earth is about 6 Rg (ronnagrams). The mass of Jupiter is about 2 Qg (quettagrams).
Examples of powers of units with metric prefixes
1 km2 means one square kilometre, or the area of a square of 1000 m by 1000 m. In other words, an area of 1000000 square metres and not 1000 square metres. 2 Mm3 means two cubic megametres, or the volume of two cubes of 1000000 m by 1000000 m by 1000000 m, i.e. 2×1018 m3, and not 2000000 cubic metres (2×106 m3).
Examples with prefixes and powers 5 mV × 5 mA = 5×10−3 V × 5×10−3 A = 25×10−6 V⋅A = 25 μW. 5.00 mV + 10 μV = 5.00 mV + 0.01 mV = 5.01 mV. 5 cm = 5×10−2 m = 5 × 0.01 m = 0.05 m. 9 km2 = 9 × (103 m)2 = 9 × (103)2 × m2 = 9×106 m2 = 9 × 1000000 m2 = 9000000 m2. 3 MW = 3×106 W = 3 × 1000000 W = 3000000 W.
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