The SI System of Units Explained

What the SI is

The SI, short for the International System of Units (from the French Système international d’unités), is the modern form of the metric system. It was formally established by the General Conference on Weights and Measures in 1960 and has been maintained and updated since. Its purpose is simple: to give every country, laboratory and factory one agreed set of units, so that a measurement made in one place means exactly the same thing in another.

The system has a clear structure. It starts with seven base units, builds all other units from them, and uses a set of prefixes to express very large and very small amounts. Understanding that structure explains most of what a unit converter does.

The seven base units

Every SI unit is built from seven base units, each tied to a different physical quantity:

• second (s) for time
• metre (m) for length
• kilogram (kg) for mass
• ampere (A) for electric current
• kelvin (K) for thermodynamic temperature
• mole (mol) for amount of substance
• candela (cd) for luminous intensity

Four of these, the second, metre, kilogram and kelvin, cover the categories most people meet every day: time, length, mass and temperature. The other three matter mostly in electricity, chemistry and lighting.

Units defined by constants of nature

Until recently, the kilogram was defined by a physical object, an international prototype kept in France, and other units depended on practical procedures. In May 2019 the definitions changed. Now each base unit is defined by fixing the exact numerical value of a constant of nature. The second is defined by the caesium-133 frequency of 9,192,631,770 hertz, and the metre by the speed of light, exactly 299,792,458 metres per second.

The kilogram is defined through the Planck constant, 6.62607015 × 10⁻³⁴ joule seconds. The ampere comes from the elementary charge, the kelvin from the Boltzmann constant of 1.380649 × 10⁻²³ joules per kelvin, and the mole from the Avogadro constant of 6.02214076 × 10²³ per mole. The candela is tied to a luminous efficacy of 683 lumens per watt for light at a specified frequency. The advantage is stability: these definitions cannot drift or be damaged, and anyone with suitable equipment can realise them.

Derived units and coherence

All other SI units are derived from the base units by multiplication and division. Speed is metres per second. Area is square metres. The newton, the unit of force, is one kilogram metre per second squared, and the joule, the unit of energy, is one newton metre. Many derived units have special names and symbols, such as the hertz for frequency, the pascal for pressure and the watt for power.

The system is described as coherent, which means that derived units follow from the base units without extra numerical factors. If you multiply a force in newtons by a distance in metres, you get energy in joules directly. Older systems often needed conversion constants at every step, which is a common source of errors.

Prefixes and units accepted alongside the SI

The SI scales units up and down with prefixes based on powers of ten. A kilometre is 10³ metres, a millimetre is 10⁻³ metres and a megametre would be 10⁶ metres. The same prefix works with any unit, so there is no need to learn a separate set of names for each quantity.

Some units are not part of the SI but are officially accepted for use with it because they are so common. The litre (1 L = 1 dm³ = 0.001 m³), the hectare (10,000 m²), the tonne (1,000 kg), the minute, the hour and the day all fall into this group. The degree Celsius is a special case: it is a named SI unit equal in size to the kelvin, but its scale starts at a different point, so 0 °C is 273.15 K.

Why the SI matters for conversion

Because SI units are defined precisely and related by exact factors, conversions to and from them are reliable. Customary units are tied to SI in the same way. The inch is exactly 25.4 millimetres, and the avoirdupois pound is exactly 0.45359237 kilograms, as fixed by the international agreement of 1959. Converting a 12-inch ruler therefore gives exactly 304.8 millimetres, with no uncertainty in the factor itself.

This is also why converters often work through a base unit: every unit in a category is expressed as a multiple of one reference unit, and any pair can be converted by going through it. For length that reference is the metre, for mass the kilogram, and for temperature the kelvin.

Using SI sensibly

Writing SI units correctly avoids confusion. Symbols are written without a full stop and are not pluralised, so write 5 km, not 5 kms. Capital and lowercase letters matter: the symbol for kelvin is a capital K, and a kilogram is lowercase kg. A space separates the number from the unit.

If you need to check a value quickly, a unit converter such as this one can translate between SI units and customary ones. For scientific, medical or engineering documents, though, rely on the standard or specification that applies, since it states the exact definitions and the required rounding.