Atto-

This article is about the SI prefix. For other uses, see Atto (disambiguation).

Look up atto- in Wiktionary, the free dictionary.

Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000000000000000001.

The unit multiple was adopted at the 12th General Conference on Weights and Measures (CGPM) in Resolution 8.[1] It is derived from the Danish word atten, meaning "eighteen".

Examples of its use:

* Weight of the HIV-1 virus is about 1×10−18 kg or 1×10−15 g, which can be written as 1 fg or 1000 ag.
* More examples available.

SI prefixes

* v
* t
* e

Prefix | Base 10  | Decimal | English word  | Adoption[nb 1] | Etymology

|
|
|
|
|

Name | Symbol | Short scale | Long scale | Language | Source word
yotta | Y | 1024 | 1000000000000000000000000 | septillion | quadrillion | 1991 | Latin | eight[nb 2]
zetta | Z | 1021 | 1000000000000000000000 | sextillion | trilliard | 1991 | Latin | seven[nb 2]
exa | E | 1018 | 1000000000000000000 | quintillion | trillion | 1975 | Greek | six
peta | P | 1015 | 1000000000000000 | quadrillion | billiard | 1975 | Greek | five[nb 2]
tera | T | 1012 | 1000000000000 | trillion | billion | 1960 | Greek | four,[nb 2] monster
giga | G | 109 | 1000000000 | billion | milliard | 1960 | Greek | giant
mega | M | 106 | 1000000 | million | 1873 | Greek | great
kilo | k | 103 | 1000 | thousand | 1795 | Greek | thousand
hecto | h | 102 | 100  | hundred | 1795 | Greek | hundred
deca | da | 101 | 10  | ten | 1795 | Greek | ten
| 100 | 1 | one | –
deci | d | 10−1 | 0.1 | tenth | 1795 | Latin | ten
centi | c | 10−2 | 0.01 | hundredth | 1795 | Latin | hundred
milli | m | 10−3 | 0.001 | thousandth | 1795 | Latin | thousand
micro | μ[nb 3] | 10−6 | 0.000001 | millionth | 1873 | Greek | small
nano | n | 10−9 | 0.000000001 | billionth | milliardth | 1960 | Greek | dwarf
pico | p | 10−12 | 0.000000000001 | trillionth | billionth | 1960 | Spanish | peak, a little bit
femto | f | 10−15 | 0.000000000000001 | quadrillionth | billiardth | 1964 | Danish | fifteen, Fermi[nb 4]
atto | a | 10−18 | 0.000000000000000001 | quintillionth | trillionth | 1964 | Danish | eighteen
zepto | z | 10−21 | 0.000000000000000000001 | sextillionth | trilliardth | 1991 | Latin | seven[nb 2]
yocto | y | 10−24 | 0.000000000000000000000001 | septillionth | quadrillionth | 1991 | Latin | eight[nb 2]

1. ^ Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.
2. ^ a b c d e f Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (source word).
3. ^ The abbreviation mcg is often used in medicine instead of μg to prevent confusion with mg, which can lead to overdoses. A lower case u is often used instead of μ where the latter character is unavailable.
4. ^ The fermi was introduced earlier with the symbol "fm", which prompted the reinterpretation of the "f" as a prefix to "m", with femto being derived from the Danish word femten due to its similarity.

## References[edit]

1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16

*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template