# Metric prefixes words Explained

Some you will be familiar with others not so, here we explain then all with examples of use.

# Yotta-

**Yotta** is the largest unit prefix in the International System of Units (SI), denoting a factor of 10^{24} or1000000000000000000000000. It has the unit symbol **Y**.

The prefix name is derived from the Greek οκτώ (*októ*), meaning *eight*, because it is equal to 1000^{8}. It was added to the SI in 1991.

Before the official adoption of the yotta prefix, the multiple was also known as *otta*.

Usage examples:

- The mass of the Earth is 5973.6 Yg or 5.9736 Zt.
- The mass of the oceans is about 1.4 Yg.
- The mass of the Moon is approximately 73.49 Yg.
- The total power output of the Sun is approximately 385 YW.
- The observable universe is estimated to be 880 Ym in diameter.
- One yottabyte is a unit of information or computer storage equal to one septillion bytes

# Zetta-

**Zetta** is a unit prefix in the metric system denoting a factor of 10^{21} or 1000000000000000000000. The prefix was added to the International System of Units (SI) in 1991 and has the symbol **Z**.

Zeta is evocative of the French numeral *sept*, meaning *seven*. Therefore it was used to denote the seventh power of 1000.

A prefix of the same value, *hepta*, was informally introduced a few years before the promulgation of zetta. It was formed from the Greek ἑπτά, (*hepta*), also meaning *seven*. Hepta never received official sanction and is obsolete. **Zeta** (Ζήτα) is the 7th letter of the Greek alphabet.

Examples:

- The mass of Earth's atmosphere is approximately 5 zettagrams (Zg).
- The volume of seawater in the Earth's oceans is approximately 1.369 zettalitres (ZL).
- Avogadro's constant is 602.2 zetta mol
^{−1}. - A zettabyte is a quantity of information or information storage capacity equal to 10
^{21}bytes or 1,000 exabytes

# Exa-

**Exa** is a unit prefix in the metric system denoting 10^{18} or 1000000000000000000. It was added to the International System of Units (SI) in 1975, and has the unit symbol **E**.

*Exa* comes from the Greek ἕξ, used as a prefix ἑξά-, meaning *six* (like hexa-), because it is equal to 1000^{6}.

Examples:

- Multiplying the 10240 megabytes of storage each Gmail user has (as of April 2012) with the estimated 260 million Gmail users, the total storage needed for Gmail is more than an exabyte (not considering backups and how Google might compress the stored data).
- 1 EeV = 10
^{18}electronvolts = 0.1602 joule - 1 exasecond is approximately 32 billion years
- 1 exametre is approximately 110 light years
- 0.43 Es ≈ the approximate age of the Universe
- 1.6 Em — 172 ± 12.5 light years — Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)

# Peta-

**Peta** is a unit prefix in the metric system denoting multiplication by 10^{15} (1000000000000000). It was adopted in the International System of Units in 1975, and has the symbol **P**.

*Peta* is derived from the Greek πέντε, meaning "five". It denotes the fifth power of 1000 (1000^{5}). It is similar to the prefix penta ("five"), but without the letter *n* (on the analogy of the prefix tera (from the Greek for "monster") for 1000^{4} looking like tetra- ("four") with a letter missing).

Examples:

- 1 petametre = 10
^{15}metres - 1 petasecond = 10
^{15}seconds - 1 petabyte = 10
^{15}bytes - the mass–energy equivalence is 89.9 PJ/kg
- 1 light-year = 9.461 Pm

# Tera-

**Tera** is a unit prefix in the metric system denoting multiplication by 10^{12} or 1000000000000. It has the symbol **T**. *Tera* is derived from Ancient Greek τέρας (teras), meaning “monster”. The prefix tera was confirmed for use in the SI in 1960.

# Giga-

**Giga** (/ˈɡɪɡə/ or /ˈdʒɪɡə/) is a unit prefix in the metric system denoting a factor of a (short-form) billion (10^{9} or 1000000000). It has the symbol **G**.

*Giga* is derived from the Greek γίγας, meaning 'giant'. The Oxford English Dictionary reports the earliest written use of *giga* in this sense to be in the Reports of the IUPAC 14th Conference in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 10^{9}×".

When referring to information units in computing, such as gigabit or gigabyte, *giga* may sometimes mean 1073741824 (2^{30}), although such use is contrary to standards and has been discouraged by the standards organizations.The binary prefix gibi has been adopted for 2^{30}, while reserving giga exclusively for the metric definition.

## Pronunciation

In English, the initial *g* of *giga* can be pronounced /ɡ/ (a hard *g* as in *giggle*), or /dʒ/ (a soft *g* as in *giant*, which shares its Greek root).

This latter pronunciation was formalized within the United States in the 1960s and 1980s with the issue by the US National Bureau of Standards of pronunciation guides for the metric prefixes.A prominent example is found in the pronunciation of*gigawatts* in the 1985 movie *Back to the Future*.

According to the American writer Kevin Self, a German committee member of the International Electrotechnical Commission proposed *giga* as a prefix for 10^{9} in the 1920s, drawing on a verse by the humorous poet Christian Morgenstern that appeared in the third (1908) edition of *Galgenlieder* (Gallows Songs). This suggests that a hard German [ɡ] was originally intended as the pronunciation. Self was unable to ascertain at what point the alternative pronunciation came into occasional use, but claimed that as of 1995 it had died out.

In 1998, a poll by the phonetician John C. Wells found that 84% of Britons preferred the pronunciation of *gigabyte* starting with /ɡɪ/ (as in *gig*), 9% with /dʒɪ/ (as in *jig*), 6% with /ɡaɪ/ (*guy*), and 1% with /dʒaɪ/ (as in *giant*).

## Common usage

- gigabyte—for instance, for hard disk capacity, 120 GB = 120000000000bytes; for file sizes 1 GB is often 1073741824 or 2
^{30}bytes, more properly termed a gibibyte (GiB) to eliminate ambiguity. The difference is rooted in base 10 and base 2 enumeration, and is also the reason why hard drives advertised as a certain capacity have less than that capacity when installed (as hard drives are advertised in*giga*bytes or*tera*bytes (base 10) whereas computers format data in gibibytes or tebibytes (base 2, powers of 2). See Hard disk units of storage capacity. - gigahertz—clock rate of a CPU, for instance, 3 GHz = 3000000000Hz
- gigabit—bandwidth of a network link, for instance, 1 Gbit/s = 1000000000bit/s.
- gigayear or gigaannum—one billion (10
^{9}) years, sometimes abbreviated Gyr, but the preferred usage is Ga.

# Mega-

**Mega** is a unit prefix in the metric system denoting a factor of one million (10^{6} or 1000000). It was confirmed for use in the International System of Units (SI) in 1960. *Mega* comes from the Greek μέγας, meaning *great*.

## Other common examples of usage

- megapixel - 1 million pixels in a digital camera
- one megatonne (of TNT) (a unit often used in measuring the explosive power of nuclear weapons) is the approximate energy released on igniting one million tonnes of TNT.
- megahertz — frequency of electromagnetic radiation for radio and television broadcasting, GSM, etc. 1 MHz = 1,000,000 Hz.

## Exponentiation

When units occur in exponentiation, such as in square and cubic forms, any size prefix is considered part of the unit, and thus included in the exponentiation.

- 1 Mm
^{2}means one square megametre or the size of a square of 1000000m by 1000000m or 10^{12}m^{2}, and not1000000square metres (10^{6}m^{2}). - 1 Mm
^{3}means one cubic megametre or the size of a cube of 1000000m by 1000000m by 1000000m or 10^{18}m^{3}, and not1000000cubic metres (10^{6}m^{3})

## Computing

In computing, *mega* may sometimes denote 1,048,576 (2^{20}) of information units (example: a megabyte, a megaword), but denotes 1000000 (10^{6}) units of other quantities, for example, transfer rates: 1megabit/s = 1000000bit/s. The prefix *mebi* has been suggested as a prefix for 2^{20} to avoid ambiguity.

# Kilo-

**kilo** is a unit prefix in the metric system denoting multiplication by one thousand. It has been used in the International System of Units where it has the unit symbol **k**, in lower case.

The prefix *kilo* is derived from the Greek word χίλιοι (*chilioi*), meaning "thousand". It was originally adopted by Antoine Lavoisier's research group in 1795, and introduced into the metric system in France with its establishment in 1799.

Examples:

- one kilogram is 1000 grams
- one kilometer is 1000 metres
- one kilojoule is 1000 joules
- one kilobaud is 1000 bauds
- one kilobit is 1000 bits
- one kilobyte is 1000 bytes.

A second definition has been in common use in some fields of computer science and information technology, which is, however, inconsistent with the SI. It uses kilo as meaning 2^{10} = 1024, because of the mathematical coincidence that 2^{10} is approximately 10^{3. The NIST comments on this confusion: "Faced with this reality, the IEEE Standards Board decided that IEEE standards will use the conventional, internationally adopted, definitions of the SI prefixes."}

- one kilohertz is 1000 hertz

## Exponentiation

When units occur in exponentiation, such as in square and cubic forms, any multiplier prefix is considered part of the unit, and thus included in the exponentiation.

- 1 km
^{2}means one square kilometre or the area of a square of 1000 m by 1000 m or 10^{6}m^{2}. - 1 km
^{3}means one cubic kilometre or the volume of a cube of 1000 m by 1000 m by 1000 m or 10^{9}m^{3}.

**Hecto-**

**Hecto** or **hecta** is a unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν *hekaton*, meaning *hundred*. Its unit symbol in the International System of Units (SI) is the lower case letter **h**.

It is rarely used, except in certain specific applications:

- hectopascal (hPa), in meteorology, for atmospheric pressure, the modern equivalent of the traditional millibar.
- hectolitre (hl or hL), in agriculture, for liquids (notably wine and milk) and bulk commodities (
*e.g.*grain). - hectogram (hg), in agronomy, for quantities of animal feed (hectogram/animal) and for measures of agricultural productivity (hectogram/hectare); also used in Italy abbreviated as
*etto*, and in Canada and New Zealand simply as 100 g, for retail sale of cold cuts and meat. - hectometre (hm), in radio astronomy, occasionally used to indicate a radio band by wavelength
- hectare (ha), in surveying, as a measure of land area equal to 10,000 m
^{2}or one square hectometre (100*ares*)

# Deca-

**Deca-** or **deka-** (symbol **da**) is a prefix in the metric system, also a numerical prefix, denoting a factor of ten. The term is derived from the Greek *δέκα* meaning "ten". Its symbol is the only SI prefix that uses more than one character.

The prefix was a part of the original metric system in 1795. It is not in very common usage, although the decapascal is occasionally used by audiologists. The decanewton is also encountered occasionally, probably because it is an SI approximation of the kilogram-force. Its use is more common in Central Europe: in German, Czech, Slovak and Hungarian, *deca* is common (and always means *decagram*).

Before the symbol was standardized as da with the introduction of the International System of Units in 1960, various other symbols were more common, such as "dk", "D", and "Da".

- 1 decametre = 10 metres
- 1 decalitre = 10 litres
- 1 decare = 10 ares

# Deci-

**Deci** (symbol **d**) is a prefix in the metric system denoting a factor of one tenth. Adopted in 1795, the prefix comes from the Latin*decimus*, meaning *tenth*. Its most frequent use is in the non-SI unit, the decibel, used to measure sound intensity (relative to a reference) and many other ratios.

# Centi-

**Centi** (symbol **c**) is a prefix in the metric system denoting a factor of one hundredth. Adopted in 1795, the prefix comes from the Latin *centum*, meaning *hundred*. It is mainly used with the metre to express lengths in *centimetres*, a common unit of length.

# Milli-

**Milli** (symbol **m**) is a prefix in the metric system denoting a factor of one thousandth (10^{−3}). Adopted in 1795, the prefix comes from the Latin *mille*, meaning *one thousand* (the plural is *milia*).

# Micro-

**Micro** (**µ**) is a prefix in the metric system denoting a factor of 10^{-6} (one millionth).Confirmed in 1960, the prefix comes from the Greek μικρός (*mikrós*), meaning "small".

The symbol for the prefix comes from the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet.

## Symbol encoding in character sets

For historical reasons, there are two different characters in Unicode, which appear slightly different in some fonts. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1985), at `U+00B5`

(Unicode 1.0, 1991). The Greek letter (μ) is encoded in the Greek range at `U+03BC`

. In circumstances in which only the Latin alphabet is available, the prefix can (unofficially) be represented using the letter u as in *um* for µm, or *uF* for µF.

In pharmacy, nursing, and other health care fields, most especially in hospital environments, the standard symbol for microgram, "µg", is deprecated because of the risk of misdose via the misreading of poor handwriting. The two alternatives are to abbreviate as "mcg" or to write out "microgram" in full, the latter being best practice (see List of abbreviations used in medical prescriptions). In physical sciences academia, "µg" remains the sole official abbreviation

# Nano-

**Nano-** (symbol **n**) is a prefix meaning a billionth. Used primarily in the metric system, this prefix denotes a factor of 10^{−9} or0.000000001. It is frequently encountered in science and electronics for prefixing units of time and length, such as 29 nanoseconds (symbol ns), 100 nanometres (nm) or in the case of electrical capacitance, 100 nanofarads (nf). The prefix is derived from the Greek νᾶνος, meaning "dwarf", and was officially confirmed as standard in 1960.

In the United States, the use of the nano prefix for the farad unit of electrical capacitance is uncommon; capacitors of that size are more often expressed in terms of a small fraction of a microfarad or a large number of picofarads.

When used as a prefix for something other than a unit of measure, as in "nanoscience", **nano** means relating to nanotechnology, or on a scale of nanometres.

# Pico-

**Pico-** (symbol **p**) is a prefix in the metric system denoting a factor of 10^{−12} or 0.000000000001.

Derived from the Italian *piccolo*, meaning *small*, this was one of the original 12 prefixes defined in 1960 when the International System of Units was established.

# Femto-

**Femto-** (symbol **f**) is a prefix in the metric system denoting a factor of 10^{−15} or 0.000000000000001. Adopted by the 11th*Conférence Générale des Poids et Mesures*, it was added in 1964 to the SI.It is derived from the Danish word *femten*, meaning "fifteen".

Example of use:

- a proton has a diameter of about 1.6 to 1.7 femtometres.

The *femtometre* shares the unit symbol (fm) with the older non-SI unit *fermi*, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.

# Atto-

**Atto-** (symbol **a**) is a prefix in the metric system denoting a factor of 10^{−18} or 0.000000000000000001.

The unit multiple was adopted at the 12th Conférence Générale des Poids et Mesures (CGPM) in Resolution 8. It is derived from the Danish word *atten*, meaning "eighteen".

# Zepto-

**Zepto-** (symbol **z**) is a prefix in the metric system denoting a factor of 10^{−21} or 0.000000000000000000001.

Adopted in 1991, it comes from the French *sept* or Latin *septem*, meaning "seven", since it is equal to 1000^{−7}.

**Examples of its use:**

- The charge on one electron: 160.217657 zeptocoulombs (zC).

- One zeptomole (zmol) of substance contains 602 particles.

# Yocto-

**Yocto-** (symbol **y**) is a prefix in the metric system denoting a factor of 10^{−24} or 0.000000000000000000000001.

It was adopted in 1991 by the General Conference on Weights and Measures. It comes from the Greek οκτώ, meaning "eight", because it is equal to 1000^{−8}. As of 2012, yocto is the smallest official SI prefix.

It can be used to state a subatomic particle's mass, for example:

Rest mass of electron: 0.000911 yg Rest mass of proton: 1.6726 yg Rest mass of neutron: 1.6749 yg Atomic mass unit: 1.6605 yg