The prefixes of the International System are used to name the multiples and submultiples of any SI unit, whether they are basic or derived units. These prefixes are prepended to the name of the unit to indicate the decimal multiple or sub-multiple of the same. Likewise, prefix symbols are prepended to unit symbols. The prefixes belonging to the SI are officially set by the International Bureau of Weights and Measures (BIPM).

Prefix Name | Symbol | Base 10 | Decimal |
---|---|---|---|

yotta | Y | 10^{24} |
1000000000000000000000000 |

zetta | Z | 10^{21} |
1000000000000000000000 |

exa | E | 10^{18} |
1000000000000000000 |

peta | P | 10^{15} |
1000000000000000 |

tera | T | 10^{12} |
1000000000000 |

giga | G | 10^{9} |
1000000000 |

mega | M | 10^{6} |
1000000 |

kilo | k | 10^{3} |
1000 |

hecto | h | 10^{2} |
100 |

deca | da | 10^{1} |
10 |

10^{0} |
1 | ||

deci | d | 10^{−1} |
0.1 |

centi | c | 10^{−2} |
0.01 |

milli | m | 10^{−3} |
0.001 |

micro | μ | 10^{−6} |
0.000001 |

nano | n | 10^{−9} |
0.000000001 |

pico | p | 10^{−12} |
0.000000000001 |

femto | f | 10^{−15} |
0.000000000000001 |

atto | a | 10^{−18} |
0.000000000000000001 |

zepto | z | 10^{−21} |
0.000000000000000000001 |

yocto | y | 10^{−24} |
0.000000000000000000000001 |

Yocto-

octo- (international symbol: y) - A standard unit prefix in the International System of Units (SI), meaning a factor of 10^{−24} or 0.000000000000000000000001. It is used together with metric and some other units. As a prefix, yocto was adopted by the XIX General Conference on Weights and Measures in 1991.

Zepto-

Zepto (symbol: 'z') is a prefix of the SI units denoting a factor of 10^{−21} or 1/1000000000000000000000. It was incorporated into the system in 1991 by the XIX General Conference on Weights and Measures. Zepto comes from the French word 'sept' and Latin word 'septem', meaning '7', since a zepto is equal to 1000^{-7}.

Atto-

Atto- (Symbol: 'a') is the prefix of the SI system, denoting one quintillionth or 10^{−18}. Examples: one attosecond or one attometer. The prefix was approved in 1964. Derived from the Danish word 'atten', which means 'eighteen'.

As an example, the kilo prefix multiplies the unit by 1000. So a kilometer equals 1000 meters and a kilowatt equals 1000 watts. The prefix milli divides the unit by one thousand. A millimeter is a thousandth of a meter, there are 1000 millimeters in a meter, and a milliliter is a thousandth of a liter. The ability to use the same prefix with different SI units is one of the great strengths of the SI system, as it greatly simplifies system learning.

Thanks to the prefixes, the value of a quantity with its own unit, in the range from yocto to yotta, can always be expressed with a number between 1 and 1000. For quantities with a unit without its own name, expressed in a power of a unit, a larger number may be unavoidable, eg 123,000,000 m³. With a product of powers of units you have the choice of which unit or units a prefix applies to. Large numbers can be avoided when appropriate by using the prefix on a unit, preferably on a unit with power 1 or −1. For example: 37 m³ / μs is shorter than 37,000,000 m³ / s. However, useful units depend on more factors. For example, the choice of unit affects accuracy (number of significant digits). In the example, a value of 37 m³ / μs is between 36.5 and 37.5 m³ / μs, while a value of 37,000,000 m³ / s is between 36,999,999.5 and 37,000,000.5 m³ / s lies.

Standardized prefixes were invented as early as Napoleon. The names for the enlarging prefixes were taken from Greek and for the diminishing prefixes from Latin. This resulted in the last four lines from the table above and myria for 10,000.

When that turned out to be insufficient, some sprawl arose. Examples are micron µ and ångström Å for one millionth and ten billionth meter.

In the second half of the twentieth century, it was therefore decided to expand the list of standard prefixes and only allow these prefixes. The micron thus became micrometer (µm) and the angstrom became 0.1 nm. By analogy, it is now also possible to make microjoules and micronewton.

Names and prefixes written in full follow the rules of the language in which it is written. Small variations are no problem because they do not cause confusion.

In Dutch, fully written prefixes and units begin with a lowercase letter (except at the beginning of a sentence): picofarad, milligrams, centimeters, kilovolts, megabytes, gigawatts, volts, amps. Other rules may apply in other languages, including German.

The prefix and unit are written together as one word. Sometimes a letter, such as in hectares, officially allowed, and megohm, does not expire according to the rules. It is also possible that the rules of the language require the use of a hyphen: mega-ampere.

The symbols for units and prefixes are written according to international rules, so independent of the language. This is important because a symbol consists of only a few letters: a small spelling difference results in a different symbol.

The symbols of the prefixes have a small letter up to and including kilo, even at the beginning of a sentence, although this is rare, and above that a capital letter: pF, mg, cm, kV, Mb, GW. If you do this incorrectly, it can cause a big difference in meaning: the notation mW means milliwatt and MW megawatt.