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Answer: SI prefixes are names and symbols added to a unit to scale it by powers of ten, such as kilo (1000), centi (one-hundredth) and milli (one-thousandth). Twenty-four prefixes are currently recognized, ranging from 10^30 to 10^-30; the newest, including ronna and quetta, were added in 2022.

Common prefixes

| Prefix | Symbol | Factor | |---|---|---| | giga | G | 10^9 | | mega | M | 10^6 | | kilo | k | 10^3 | | hecto | h | 10^2 | | deka | da | 10 | | deci | d | 10^-1 | | centi | c | 10^-2 | | milli | m | 10^-3 | | micro | µ | 10^-6 | | nano | n | 10^-9 |

History

Eight original prefixes were adopted in 1795, from Greek and Latin numbers. In 1960, myria and myrio were dropped and mega, giga, tera, micro, nano and pico were added. Later expansions continued to 2022.

Writing tips from NIST

• Symbols are case-sensitive: M is mega, m is milli.
• In the U.S., use the dot as the decimal marker and a leading zero for numbers below one (0.5 m).

The bigger system

Prefixes attach to the SI's 7 base units and 22 named derived units, which rest on seven defining constants.

Claims

• id: C1 claim: Prefixes deka, hecto and kilo mean 10, 100 and 1000. source: S1 evidence: "the prefixes deka, hecto, and kilo meaning, respectively, 10, 100, and 1000"
• id: C2 claim: In 1960 myria and myrio were made obsolete. source: S1 evidence: "1960Two prefixes were made obsolete: myria and myrio."
• id: C3 claim: Use a leading zero for numbers less than one. source: S1 evidence: "Use a leading zero for numbers less than one"
• id: C4 claim: Twenty-four prefixes are currently recognized. source: S2 evidence: "Twenty-four prefixes ranging from 1030 to 10−30, are currently recognized for use."
• id: C5 claim: The SI rests on seven defining constants. source: S2 evidence: "The SI rests on a foundation of seven (7) defining constants"

Sources

• [S1] Metric (SI) Prefixes. NIST. https://www.nist.gov/pml/owm/metric-si-prefixes . License: US government work, public domain. Accessed 2026-11-04.
• [S2] SI Units. NIST. https://www.nist.gov/pml/weights-and-measures/metric-si/si-units . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_rwyae7g0,
  author = {{iinventedlight}},
  title = {{Metric (SI) prefixes}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/metric-si-prefixes/},
  note = {Permanent ID iil:note:rwyae7g0. License: CC-BY-4.0.},
}

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