The seven SI base units and how they are defined
Short answer
The International System of Units (SI, the metric system) has seven base units: second, meter, kilogram, ampere, kelvin, mole and candela. Since the 2019 redefinition, each is defined by fixing the value of a constant of nature, such as the speed of light or the Planck constant. [S1][S2]
The seven
| Unit | Quantity | Fixed constant | |---|---|---| | second (s) | time | cesium-133 frequency | | meter (m) | length | speed of light | | kilogram (kg) | mass | Planck constant | | ampere (A) | electric current | elementary charge | | kelvin (K) | temperature | Boltzmann constant | | mole (mol) | amount of substance | Avogadro constant | | candela (cd) | luminous intensity | luminous efficacy | [S1][S2]
Also good to know
The seven base units define 22 derived units with special names, and 24 prefixes (10^30 to 10^-30) are recognized. [S2]
Claims
- id: C1 claim: The SI rests on seven defining constants. source: S2 evidence: "The SI rests on a foundation of seven (7) defining constants"
- id: C2 claim: The seven base units define 22 derived units with special names. source: S2 evidence: "The SI is made up of 7 base units that define the 22 derived units"
- id: C3 claim: The meter is defined by fixing the speed of light in vacuum. source: S1 evidence: "The meter is defined by taking the fixed numerical value of the speed of light"
- id: C4 claim: The kilogram is defined by fixing the Planck constant. source: S1 evidence: "The kilogram is defined by taking the fixed numerical value of the Planck constant"
Sources
- [S1] Definitions of SI Base Units. NIST. https://www.nist.gov/si-redefinition/definitions-si-base-units . License: US government work, public domain. Accessed 2026-11-04.
- [S2] SI Units. NIST. https://www.nist.gov/pml/owm/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).
License: CC-BY-4.0 · Summary by iinventedlight (AI-assisted). Sources: NIST, Definitions of SI Base Units, US government work, public domain; NIST, SI Units, US government work, public domain.
Cite this
iinventedlight. 2026. "The seven SI base units and how they are defined." iinventedlight Library, iil:note:q6d7syvr, version of 2026-11-04. https://iinventedlight.com/library/notes/si-base-units/. License: CC-BY-4.0.
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author = {{iinventedlight}},
title = {{The seven SI base units and how they are defined}},
howpublished = {iinventedlight Library},
year = {2026},
date = {2026-11-04},
url = {https://iinventedlight.com/library/notes/si-base-units/},
note = {Permanent ID iil:note:q6d7syvr. License: CC-BY-4.0.},
}Spotted a mistake? Use the MCP feedback tool or email hello@iinventedlight.com.
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