Base and Derived Units

There are seven Base Units. These are the units on which every other unit is based.

So what exactly is a “unit”?

A unit of measurement, or “unit” for short, tells you how something has been measured. If I said that I lived one mile from school then you would have a good idea about how far it is from my house to the science lab. But if I said I lived “1” from school, without any unit of measurement, then you would be completely in the dark: I might mean a mile or a kilometre, but I could just as easily mean an hour’s drive or one day on horseback.

Bizarre though this example may sound, measurements really were made in apparently random fashions in the past and it took an international agreement for everybody to recognise one standard set of units. The seven fundamental units, listed below, are known as the SI Units. Only the first five (mass, length, time, electric current and temperature) are required for the GCSE Physics course but mention will also be made of the mole as it’s important in understanding mechanisms when we talk about things on an atomic scale. Although the base units were previously defined using physical objects, they are now all defined using the fundamental constants of nature.

We cannot measure everything using these seven units alone but we can measure everything using a specific combination of several of the base units. Some of the derived units have their own names and symbols (such as newton, N, which is the the unit of force) but the structure of other derived units reveals their connection to the base units.

A nice summary of the SI units, both base and derived, is available from the Bureau International des Poids et Mesures (BIPM): it is in English and can be read here. Beware, however, that this document still refers to physical standards rather than the new definitions based on the fundamental constants of nature (see below).

For more than 100 years there was a “master mass” that was exactly 1 kg: it had to be stored incredibly safely to ensure that it never lost any of its mass so that it always remained the perfect reference point for every other mass in the world. That really wasn’t practical and in 2018 the method of defining the base measurements changed. At GCSE level, you don’t need to worry about how the measurements are defined but if you’re interested, the BIPM’s summary of the latest definitions, published in 2019, can be read in this document.