The International System of Units is built on seven base units, each measuring a distinct physical quantity. Every other unit in the SI — from the newton to the volt — is a product of powers of these seven. This reference gives each unit’s symbol and its modern, constant-based definition.
How it works
Pick a base quantity and the tool shows its unit, symbol, and the defining relationship adopted in the 2019 SI redefinition. Before 2019 the kilogram was defined by a physical metal cylinder; since then every base unit has been fixed by an exact numerical value of a constant of nature:
- metre via the speed of light
c - kilogram via the Planck constant
h - second via the caesium-133 hyperfine frequency
- ampere via the elementary charge
e - kelvin via the Boltzmann constant
k - mole via the Avogadro constant
NA - candela via the luminous efficacy of 540 THz light
Anchoring units to constants makes them reproducible in any lab and immune to the drift that affected the old physical prototypes.
The seven base units at a glance
| Quantity | Unit | Symbol | Constant used |
|---|---|---|---|
| Length | metre | m | speed of light c |
| Mass | kilogram | kg | Planck constant h |
| Time | second | s | Cs-133 hyperfine frequency |
| 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 at 540 THz |
Why constants instead of artefacts
The original kilogram was defined as the mass of a physical platinum-iridium cylinder kept in a vault near Paris. National metrology institutes around the world compared their own national prototypes against this single object to calibrate their mass scales. The problem is that physical artefacts change over time: surface atoms can be absorbed or lost through cleaning, handling, and environmental exposure. Measurements showed that national copies had drifted relative to the Paris prototype by measurable amounts over a century — meaning the definition of mass was itself slightly unstable.
By fixing the Planck constant h to an exact value, the kilogram becomes a quantity
that can be realised from first principles anywhere with the right equipment. A Kibble
balance converts electrical and mechanical measurements into mass using fixed constants,
making the kilogram as stable as the laws of quantum mechanics.
How derived units follow
Every familiar unit in engineering and science is a combination of powers of the seven base units:
- Newton (force): kg·m·s⁻²
- Pascal (pressure): kg·m⁻¹·s⁻²
- Joule (energy): kg·m²·s⁻²
- Volt (electric potential): kg·m²·s⁻³·A⁻¹
- Hertz (frequency): s⁻¹
Understanding the base-unit composition of a derived unit helps catch dimension errors in equations: if both sides do not resolve to the same combination of m, kg, s, A, K, mol, cd, the equation is wrong.
Notes
The redefinition was invisible in everyday life — a kilogram of sugar still weighs the same — but it future-proofed metrology for the next century. The seven base units and their symbols (m, kg, s, A, K, mol, cd) are worth memorising because all derived units trace back to them.