The Seven SI Base Units and the 2019 Redefinition
In 2019 the last physical artefact defining a unit was retired. Every SI unit is now defined by a constant of nature.
18 May 2026 · 6 min read · Science
Every unit on ZapUnits ultimately traces back to seven base units — and since 2019, all seven are defined by fixed constants of nature rather than by objects.
The seven
| Quantity | Unit | Symbol | Defined by |
|---|---|---|---|
| Length | metre | m | Speed of light, c |
| Mass | kilogram | kg | Planck constant, h |
| Time | second | s | Caesium-133 hyperfine transition |
| Electric current | ampere | A | Elementary charge, e |
| Temperature | kelvin | K | Boltzmann constant, k |
| Amount of substance | mole | mol | Avogadro constant, NA |
| Luminous intensity | candela | cd | Luminous efficacy, Kcd |
The kilogram problem
Until 2019 the kilogram was a platinum-iridium cylinder in a vault near Paris — the International Prototype Kilogram, cast in 1889. Copies were distributed worldwide and compared periodically.
They drifted. By the 1990s the official copies differed from the prototype by up to 50 micrograms. Worse, there was no way to tell whether the copies were gaining or the prototype was losing, because the prototype was the definition. If it had been scratched, the mass of the universe would have been redefined by decree.
The 2019 redefinition fixed the Planck constant at exactly 6.62607015 × 10⁻³⁴ J·s and derived the kilogram from it, realisable anywhere with a Kibble balance.
Everything else is derived
Every other unit is a combination:
- newton = kg·m/s²
- joule = N·m = kg·m²/s²
- watt = J/s
- pascal = N/m²
- volt = W/A
- ohm = V/A
That is why our energy converter can move between joules, kilowatt-hours and BTU without any physical constant of its own: they are all the same combination of base units.
Prefixes
SI prefixes span 10⁻³⁰ to 10³⁰ since ronna, quetta, ronto and quecto were added in 2022. The ones you meet daily:
| Prefix | Factor | Example |
|---|---|---|
| nano | 10⁻⁹ | nanometre |
| micro | 10⁻⁶ | microgram |
| milli | 10⁻³ | millilitre |
| kilo | 10³ | kilogram |
| mega | 10⁶ | megawatt |
| giga | 10⁹ | gigabyte |
| tera | 10¹² | terabyte |
Note the capitalisation: k is kilo, K is kelvin; m is milli, M is mega. A "1 Mb" file is a thousand times bigger than a "1 mb" one.
Related reading
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy