---
source: https://iinventedlight.com/library/notes/si-base-units.md
title: "The seven SI base units and how they are defined"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "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."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "tutor-science: measurement"
tags: science, measurement, si-units, metric
sources:
  - id: S1
    title: "Definitions of SI Base Units"
    publisher: NIST
    url: https://www.nist.gov/si-redefinition/definitions-si-base-units
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "SI Units"
    publisher: NIST
    url: https://www.nist.gov/pml/owm/metric-si/si-units
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# 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).