Base units

We include all of the seven base units at the core of the SI. They are so called because each one introduces an independent physical quantity not contained in any other unit in the system. The formal definitions are given along with a lay person's guide as to what they measure (note that the formal definitions may be simplified for clarity).

SI base units
Unit name Symbol What it measures and its definition
metre m
Length or distance
The metre is a unit of distance and is the basis for measuring length, area and volume. The metre was originally derived from one ten millionth of the distance from Earth's pole to the equator but has been refined (1) several times to link it to more stable physical constants. It is now defined in terms of the distance travelled at the speed of light during a very precisely defined fraction of a second.
The length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.
kilogram kg
Mass
In ordinary language, the kilogram measures what most regard as weight. Strictly speaking, mass is the amount of matter in an object, whereas weight is to do with the pull of gravity (how heavy it is). It was originally just 1000 grams where 1 gram was the mass of 1 cubic centimetre of water (making the kilogram equal to the mass of 1 litre of water). Nowadays it is defined in terms of a much more stable reference in the form of a piece of platinum iridium known as the international prototype. For many years, scientists have been trying to find a better way of defining and measuring it in terms of natural constants in a manner similar to that of the metre and second.
A unit of mass equal to the mass of the international prototype of the kilogram.
second s
Time
It is the familiar unit of time that we count with our watches and clocks. It is now defined by the natural beats of a highly stable caesium atomic clock.
The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
ampere A
Electric current
This is the familiar 'amp' we associate with things like fuses and electric cable. It is the fundamental base unit for electricity and magnetism. It is now defined in terms of the magnetic force (induced by the current) exerted between a pair of wires one metre apart.
A constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length. (See section on derived units for a description of the newton.)
kelvin K
Thermodynamic temperature
It measures temperature in the way we normally understand it, i.e. how hot something is. It is closely linked to the Celsius scale except that zero kelvin (0 K) is when there is no heat at all (absolute zero) and everything freezes (-273.15 °C).
The fraction 1/273.16 of the thermodynamic temperature of the triple point of water (i.e. freezing point of water).
mole mol
Amount of a substance
Used in chemistry and physics. It represents a fixed number of "elementary entities" of a substance. When the mole is used, the elementary entities must be specified. Those entities may be atoms, molecules, ions, electrons or any groups of particles so long as they are all the same.
The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
candela cd
Luminous intensity
Essentially, this measures the 'brightness' of radiation. It is defined in terms of the intensity of a very pure yellow-green light source with a strength measured by its power spread over a cone shaped beam.
Luminous intensity, in a given direction, of a source that emits monochromatic radiation at a frequency of 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

(1) Please note that although the definition has changed, the actual length of the metre has not. The revised definition represents a very precise statement of the length of the original prototype that was used as the master reference for the measurement of distance world-wide. We now know it to be slightly in error in respect of the true figure of the Earth (inevitably so with the huge advancements in technology since the eighteenth century) but that is of no practical consequence. Nevertheless, the fact that the Earth's circumference is very close to 40 000 km is still handy when using globes or world maps.