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 (e.g., millidyne and milligauss). 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 (and kilotonnes, megatonnes, etc.) 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 (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids (e.g. bottles or servings of wine), and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres). Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific (other than medical) 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 (0.1 nm) 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 (e.g. millions) 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 (60 seconds),
hours (60 minutes) and
days (24 hours) 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 (a) is a standardised variant of the
year, equal to exactly seconds ( days). 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 (Ma) or
gigaannum (Ga).
Angle The SI unit of angle is the
radian, but
degrees, as well as
arc-minutes and arc-seconds, see some scientific use in fields such as astronomy.
Temperature Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states: It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal.
Non-metric units Metric prefixes are widely used outside the metric SI system. Common examples include the
megabyte and the
decibel. Metric prefixes rarely appear with
imperial or
US units except in some special cases (e.g., microinch, kilofoot,
kilopound). They are also used with other specialised units used in particular fields (e.g.,
megaelectronvolt,
gigaparsec,
millibarn,
kilodalton). In astronomy, geology, and palaeontology, the
year, with symbol 'a' (from the Latin
annus), is commonly used with metric prefixes:
ka, Ma, and Ga. Official policies about the use of SI prefixes with non-SI units vary slightly between the
International Bureau of Weights and Measures (BIPM) and the American
National Institute of Standards and Technology (NIST). For instance, the NIST advises that "to avoid confusion, prefix symbols (and prefix names) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) ° (degree), (minute), and (second), whereas the BIPM adds information about the use of prefixes with the symbol
as for arcsecond when they state: "However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles." == Non-standard prefixes ==