On 20 May 2019 the SI stopped depending on any physical object. The last artefact — a platinum-iridium cylinder in a vault near Paris that was the kilogram — was retired, and all seven base units are now defined by fixing the numerical values of seven constants of nature.
That change is invisible in daily use, by design. It matters because it makes every unit reproducible anywhere with the right equipment, rather than traceable back to one object that could be scratched, contaminated or lost.
The seven base units
| Unit | Symbol | Quantity | Defined by fixing |
|---|---|---|---|
| second | s | Time | The caesium-133 hyperfine transition frequency |
| metre | m | Length | The speed of light, c |
| kilogram | kg | Mass | The Planck constant, h |
| ampere | A | Electric current | The elementary charge, e |
| kelvin | K | Temperature | The Boltzmann constant, k |
| mole | mol | Amount of substance | The Avogadro constant, NA |
| candela | cd | Luminous intensity | The luminous efficacy Kcd |
Notice the dependency chain. The second is defined first, from an atomic transition. The metre then follows from the second and a fixed speed of light. The kilogram follows from the metre, the second and a fixed Planck constant. Each unit is built on the ones before it, which is why the second is the most precisely realised unit in existence and why improvements there propagate through everything else.
Derived units: the point of a coherent system
Every other SI unit is a product of powers of the seven base units, with no numerical factor anywhere. This property is called coherence, and it is the whole reason SI is pleasant to calculate in.
| Unit | Quantity | In base units |
|---|---|---|
| newton (N) | Force | kg·m·s⁻² |
| joule (J) | Energy | N·m = kg·m²·s⁻² |
| watt (W) | Power | J/s = kg·m²·s⁻³ |
| pascal (Pa) | Pressure | N/m² = kg·m⁻¹·s⁻² |
| volt (V) | Electric potential | W/A = kg·m²·s⁻³·A⁻¹ |
| hertz (Hz) | Frequency | s⁻¹ |
A force of one newton acting through one metre does exactly one joule of work. No conversion constant appears. Compare the Imperial equivalent, where relating pounds-mass to pounds-force requires the factor 32.174, and every such factor is a place to drop a decimal.
Prefixes
The prefixes span 10⁻³⁰ to 10³⁰ since quetta, ronna, ronto and quecto were added in 2022, largely because global data volumes were about to outgrow yotta.
| Prefix | Symbol | Factor | Prefix | Symbol | Factor |
|---|---|---|---|---|---|
| quetta | Q | 10³⁰ | deci | d | 10⁻¹ |
| ronna | R | 10²⁷ | centi | c | 10⁻² |
| yotta | Y | 10²⁴ | milli | m | 10⁻³ |
| zetta | Z | 10²¹ | micro | µ | 10⁻⁶ |
| exa | E | 10¹⁸ | nano | n | 10⁻⁹ |
| peta | P | 10¹⁵ | pico | p | 10⁻¹² |
| tera | T | 10¹² | femto | f | 10⁻¹⁵ |
| giga | G | 10⁹ | atto | a | 10⁻¹⁸ |
| mega | M | 10⁶ | zepto | z | 10⁻²¹ |
| kilo | k | 10³ | yocto | y | 10⁻²⁴ |
| hecto | h | 10² | ronto | r | 10⁻²⁷ |
| deka | da | 10¹ | quecto | q | 10⁻³⁰ |
Two things catch people out. Capitalisation is meaningful: M is
mega and m is milli, a factor of a billion apart. And prefixes
do not stack — there is no "millimicrometre"; it is a nanometre. The full
table with worked examples is on the
SI prefixes reference.
The kilogram is the odd one out: it is the only base unit whose name already contains a prefix. Prefixes therefore attach to the gram, not the kilogram — a millionth of a kilogram is a milligram, never a "microkilogram".
Writing SI correctly
NIST Special Publication 811 is the reference here. The rules that get broken most often:
- Space between number and symbol. 25 kg, not 25kg. The exceptions are °, ′ and ″ for angles.
- Symbols are never pluralised. 5 kg, not 5 kgs.
- No full stop after a symbol unless it ends a sentence.
- Symbols are lower case unless named after a person: m, s, kg — but N (Newton), Pa (Pascal), W (Watt), K (Kelvin).
- Spelled-out names are lower case even when they honour someone: "a force of ten newtons".
- The kelvin takes no degree sign. 300 K, not 300 °K.
- Degree Celsius takes a space and a degree sign: 25 °C.
- Use a middle dot or a space for products (N·m or N m) and a solidus or negative exponent for quotients (m/s or m·s⁻¹). Never two solidi: m/s², not m/s/s.
Non-SI units that are allowed anyway
The SI Brochure accepts a short list of non-SI units for use alongside SI, because they are too entrenched to displace: the minute, hour and day; the degree, minute and second of angle; the litre; the tonne; the astronomical unit; the electronvolt; and the dalton. The bar and the hectare are tolerated in specific fields.
Everything else — the pound, the inch, the psi, the horsepower — sits outside SI, and every one of them is now defined through SI, which is why the conversions this site performs are exact rather than measured.
Why the 2019 redefinition happened
The international prototype kilogram was compared periodically against its official copies, and the comparisons suggested a drift of a few tens of micrograms over a century. Nobody could say whether the prototype had gained mass or the copies had lost it, because by definition the prototype was always exactly one kilogram. That circularity was the problem.
Fixing the Planck constant removed it. A Kibble balance — which relates mechanical and electrical power — can now realise the kilogram from first principles in any suitably equipped laboratory. The same reasoning applied to the ampere, kelvin and mole, all of which had definitions that were awkward to realise in practice.
The numerical values were chosen so that nothing changed measurably at the moment of transition. That was deliberate: a redefinition that shifted everyday values would have been unusable.