Ever wondered why people use inconvenient forms of measurement for simple things? Take one foot. It consists of twelve inches, while three feet make a yard and 1760 yards make a mile. These numbers, based off traditional ways of measuring things, aren’t very easy to calculate with, and for this reason the metric system was introduced at the end of the eighteenth century by French Revolutionaries.

So how exactly does the metric system work? Well, the system is based off the convenience of using multiples of ten in calculations, hence we have ten centimetres in a decimetre, ten decimetres in a metre and so on. But it’s not just for lengths. The modern version of the metric system – what the original 18th century version evolved into – is the International System of Units. In French, this is *Système international d’unités* which is where we get the abbreviation of SI from. There are seven base units in the SI system:

- Metre, symbol m, used for measuring length
- Kilogram, kg, used for measuring mass (not weight! But that’s a topic for another post)
- Second, s, used for measuring time
- Ampere (often shortened to amp), A, used for measuring electric current
- Kelvin, K (not °K!), used for measuring temperature
- Mole, mol, used to measure amount of substance (this will be familiar with students of chemistry)
- Candela, cd, used to measure luminous intensity

(Interesting fact: sharp-eyed readers may have noticed that some units are capitalised, like A for Amperes or K for Kelvin, while others are not, like m for metres. Units which are named after people (usually famous scientists), have capital letters, while the others don’t)

Seven units may not seem that much at first – what if you want tomeasure something which isn’t on the list above, like area or electric charge or force? Well, the true beauty of the SI system is that *every* *single* other unit used in science can be derived by combining base units using multiplication or division. For example, area is measure in m^{2 } and electric charge is measured in Coulombs, where 1 C = 1 As (Amps times seconds). Force is a little more complicated (for the record, force is measured in Newtons: 1 Newton is 1 kgms^{-2}. This comes from Newton’s second law of motion, which I’m hoping to cover soon). So whatever you want to measure, you can measure it in SI units

However, all this doesn’t answer the question of why anybody would want to use SI units instead of imperial units. The answer to that lies, as mentioned above, in the ease of calculation that comes from using powers of ten. You see, in front of any unit, you can put a prefix which tells you to multiply it by a certain power of ten. This is the difference between, say, a gram and a kilogram, or a second and a millisecond. The most commonly used prefixes are in the list below:

### For small things:

- centi- divide by 100 (symbol c)
- milli- divide by 1,000 (m)
- micro- divide by 1,000,000 (μ)
- nano- divide by 1,000,000,000 (n)

### For big things:

- kilo- times by 1,000 (k)
- mega- times by 1,000,000 (M)
- giga- times by 1,000,000,000 (G)
- tera- times by 1,000,000,000,000 (T)

Because everything is based of multiples of ten, and every unit can be derived from just seven base units, SI does away with much of the needless complexity of the Imperial system, and is now the standard system of measuring for science and engineering (and most countries, with the biggest exception being the USA).

[…] Finally, Newton’s Second law gives us the SI unit (confused? see the post on SI units here) for force. Mass is measured in kg, while acceleration is measured in ms-2, so the unit for force […]