Metric prefix

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 by one thousand: one kilogram is equal to one thousand grams. The prefix milli, likewise, may be added to metre to indicate 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 by the International Bureau of Weights and Measures 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.

List of SI prefixes

The BIPM specifies twenty-four prefixes for the International System of Units.
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 being already used for other prefixes, already used for SI units, or easily confused with mathematical operators. 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. 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 |.
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 and hundreds are less common and are disfavoured in certain fields. Hence, 100 m is preferred over 1 hm or 10 dam. The prefixes deci- and centi-, and less frequently hecto and deca, are generally used for informal purposes; the centimetre 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 is used instead of microkilogram.
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.
The mass of 1 litre of water is about 1 kg.
The mass of the Earth is about 6 Rg.
The mass of Jupiter is about 2 Qg.
Examples of powers of units with metric prefixes
1 km² means one square kilometre, or the area of a square of by. In other words, an area of square metres and not square metres.
2 Mm³ means two cubic megametres, or the volume of two cubes of by by, i.e., and not cubic metres.
Examples with prefixes and powers
× = × = =.
+ = + =.
= = = 0.05 m.
= = = = = .
3 MW = = 3 × =.
Micro symbol

When mega and micro were adopted in 1873, three prefixes existed starting with "m". It was necessary to use a symbol other than upper and lowercase 'm'. Eventually the Greek letter "μ" was adopted.
With the lack of a "μ" key on most typewriters, as well as computer keyboards, various other abbreviations remained common, including "mc", "mic", M, and "u".
From about 1960 onwards, "u" prevailed in type-written documents. Because ASCII, EBCDIC, and other common encodings lacked code-points for "", this tradition remained even as computers replaced typewriters.
When ISO 8859-1 was created, it included the "" symbol for micro at codepoint ; later, the whole of ISO 8859-1 was incorporated into the initial version of Unicode. Many fonts that support both characters render them identically, but because the micro sign and the Greek lower-case letter have different applications, some fonts render them differently, e.g. Linux Libertine and Segoe UI.

Keyboard entry

Most English-language keyboards do not have a "" key, so it is necessary to use a key-code; this varies depending on the operating system, physical keyboard layout, and user's language.
; For all keyboard layouts
; For QWERTY keyboard layouts

Typesetting in LaTeX

The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example,

\qty formats as "3 THz".

Application to units of measurement

The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not. Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time.

Metric units

Mass

The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes or scientific notation are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".
The kilogram is the only coherent unit of the International System of Units that includes a metric prefix.

Volume

The litre, millilitre, microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre.
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres or cubic kilometres. For scientific purposes, the SI unit of cubic metre is usually used, with scientific notation rather than prefixes.

Length

The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by the older non-SI name micron, which is officially deprecated. In some fields, such as chemistry, the ångström has been used commonly instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical units, light years, and parsecs, and less commonly large multiples of kilometres; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.

Time

Prefixes for the SI standard unit second are most commonly encountered for quantities less than one second. For larger quantities, the system of minutes, hours and days is accepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardised to seconds so as not to create issues with the irregular leap second.
Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy, the Julian year or annum is a standardised variant of the year, equal to exactly seconds. The unit is so named because it was the average length of a year in the Julian calendar. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum or gigaannum.