Metric prefix

A metric prefix is a special word we add to a unit of measurement to show if we are talking about a bigger or smaller amount. For example, the word kilo can be added to gram to mean one thousand grams, which we call a kilogram. In the same way, the word milli can be added to metre to mean one thousandth of a metre, which we call a millimetre.

All metric prefixes we use today are based on ten. Each prefix has its own short symbol that we write before the unit’s symbol. These prefixes have been used in the metric system since the 1790s. Today, special prefixes called SI prefixes are used around the world in the International System of Units. These prefixes help scientists, engineers, and everyday people talk about very big or very small amounts in a clear and easy way.

List of SI prefixes

Several terms redirect here including "Femto", "Ronna-", "Yotta", and "Zepto". For the character Femto, see Griffith (Berserk). For the name, see Ronna. For the prize-linked savings account, see Yotta Technologies. For the Finnish musician, see Yotto. For similar terms, see Yota (disambiguation). For the Indian Q-commerce company, see Zepto (company). For other uses, see Atto, Exa (disambiguation), Peta, Pico, Quetta (disambiguation), Ronto, and Tera.

The International System of Units (SI) has twenty-four prefixes.

These prefixes were first used after the French Revolution. More prefixes were added over time and officially adopted in 1960.

The newest prefixes are ronna, quetta, ronto, and quecto. They were adopted in 2022. They were added for use in data science and to keep the prefixes balanced.

The prefixes from peta to quetta are based on old Greek and Latin numbers. Letters were added to avoid confusion.

Rules

• Prefix symbols are added to unit symbols. For example, kilometre is km, kilogram is kg, and kilowatt is kW. (The symbol for kilo is k.) Most prefix symbols are uppercase letters for larger amounts and lowercase letters for smaller amounts.
• All prefix symbols are Latin letters except for micro, which uses the Greek letter mu (μ).
• Prefix symbols are placed directly before the unit symbol with no space. For example, 'ms' means millisecond, while 'm s' means metre-second.
• Prefixes for multiples of one thousand are used most often. Prefixes for tens and hundreds are used less and mainly for informal purposes. For example, 100 m is preferred over 1 hm. The centimetre (cm) is common. Some rules prefer millimetres to centimetres.
• Prefixes cannot be combined on one symbol. For example, milligram (mg) is used instead of microkilogram (μkg).
• In math, prefixes are treated as multipliers. For example, 5 km is treated as 5000 m.
• A prefix symbol stays with the unit symbol when the unit is raised to a power. For example, 1 km² means 1 km × 1 km = 10⁶ m².

Usage

Metric prefixes are special labels we add to units of measurement to make them bigger or smaller. We can use these prefixes with units like grams or meters.

• 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).

When we use metric prefixes with units that are squared or cubed, it changes the meaning in special ways:

• 1 km² means one square kilometre, or the area of a square that is 1000 m on each side. This equals 1,000,000 square metres.
• 2 Mm³ means two cubic megametres, or the volume of two cubes that are 1,000,000 m on each side. This equals 2×10¹⁸ m³.

Here are some more examples showing how prefixes work with math:

• 5 mV × 5 mA = 25 μW.
• 5.00 mV + 10 μV = 5.01 mV.
• 5 cm = 0.05 m.
• 9 km² = 9,000,000 m².
• 3 MW = 3,000,000 W.

Application to units of measurement

The use of prefixes started with the metric system in the 1790s, long before the 1960 introduction of the SI. These prefixes can be used with any metric unit, whether part of the SI or not, like millidyne and milligauss. They can also sometimes be used with non-metric units, but not with non-SI units of time.

The kilogram, gram, milligram, microgram, and smaller units are commonly used for measuring mass. However, larger units like megagram and gigagram are rarely used; tonnes or scientific notation are used instead. The kilogram is the only unit in the International System of Units that includes a metric prefix.

The litre, millilitre, microlitre, and smaller are common for measuring volume. In Europe, the centilitre is often used for liquids, while the hectolitre is used for bulk products like grain or beer. Larger volumes are usually measured in kilolitres, megalitres, or gigalitres, or in cubic metres.

For measuring length, the kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often called a micron, though this name is not officially recommended. For very small lengths, the ångström and femtometre are sometimes used. Large distances are usually measured using non-metric units like the solar radius, astronomical units, light years, and parsecs.

For time, prefixes are most commonly used for fractions of a second. Larger times are usually measured in minutes, hours, and days. Very long times in science are measured using the Julian year, with prefixes like megaannum (Ma) or gigaannum.

The SI unit for angle is the radian, but degrees and arc-minutes and arc-seconds are also used, especially in astronomy.

For temperature, prefixes are more commonly used with the kelvin to show very high or very low temperatures, like MK (megakelvin) or mK (millikelvin).

The joule and kilojoule are common for measuring energy. The kilowatt-hour is often used for electrical energy. The unit calorie has different definitions, with metric prefixes usually applied to the smaller gram calorie.

Non-standard prefixes

See also: Unit prefix § Unofficial prefixes

Obsolete metric prefixes

Some old prefixes used in the metric system are no longer used. The prefix for ten thousand, myria-, and early prefixes like double- (meaning 2×) and demi- (meaning half) were part of the original system in France in 1795 but were dropped later.

Other old prefixes include hebdo- for ten million and micri- for one hundred trillionth.

Double prefixes

In the past, people used double prefixes like micromillimetres (now called nanometres) and micromicrofarads (now picofarads). These are not used with today’s standard metric system.

Other old double prefixes included "decimilli-" (one ten-thousandth), which was shortened to "dimi-" in France until 1961.

There are no more letters left in the alphabet to create new prefixes. Some ideas suggest using compound prefixes like kiloquetta- for very large numbers, but this has not been officially approved.

Similar symbols and abbreviations

In everyday English, people use the letter K to mean "thousand." For example, someone might say they have a 40K salary, meaning 40,000.

In news and money talks, m or M stands for million, b or B stands for billion (10⁹), and t or T stands for trillion (10¹²). In the medical and automotive fields in the United States, cc or ccm means cubic centimetres, which is the same as a millilitre.

Engineers used to use MCM to mean a thousand circular mils for big electrical cables, but now kcmil is the official term. In natural gas and oil, mm or MM stands for millions of British thermal units or therms, and MMbbl means millions of barrels. The letter M for thousand comes from old Roman numerals, where it means 1000.

Typography

Micro symbol

Main article: Micro-

When the prefixes mega- and micro- were first used in 1873, they needed a special symbol that was not just the letter M. They chose the Greek letter mu (μ) to show the difference.

Because many old typewriters and computer keyboards didn’t have a key for μ, people used other short ways to write it, like "mc", "mic", "M", or "u". By around 1960, "u" became the most common way to write it in typed documents. Even when computers replaced typewriters, this kept happening because common computer systems did not include the μ symbol.

Later, the standard ISO 8859-1 added the μ symbol for micro- at a special code point. This was also included in Unicode, where it is listed at U+03BC. Some fonts show this symbol and the Greek letter μ the same way, while others show them differently.

Keyboard entry

Most English-language keyboards do not have a key for μ, so people use special codes to type it. The way to do this depends on the type of computer you are using.

On Microsoft Windows computers:

• You can type it by holding Alt and pressing 0 1 8 1 on the number pad, or
• You can also type it using Alt and the codes b+5, or
• On older systems, you can type it by holding Alt and pressing 2 3 0 on the number pad;

On MacOS computers, you can type it by pressing ⌥ Opt+m or ⌥ Opt+Y.

On Linux computers:

• If you have a Compose key enabled, you can type it by pressing Compose+m+u
• With ibus version 1.5.19 or higher, you can type it by pressing Ctrl+⇧ Shift+u b 5 space
• On the VGA console, you can type it by holding Alt and pressing 1 8 1 on the number pad
• You can also type it by pressing right Alt+m (if the right alt key is set to act as AltGr).

Typesetting in LaTeX

The LaTeX system has a special package called SIunitx that helps write units of measurement clearly. For example, \qty{3}{\tera\hertz} will show up as "3 THz".