5 SI Système International or the Metric System
SI is the abbreviation of the French “Système International d’Unités”.
BIPM is the Bureau International des Poids et Mesures, The official website for this “intergovernmental organization through which Member States act together on matters related to measurement science and measurement standards” can be accessed at
https://www.bipm.org/en/measurement-units/
Since most countries in the world use this officially, including Canada, you are expected to be familiar with its use.
Prefixes
When a number is very large or very small, the units are modified using a prefix.
1 000 000 000 000 = 1012 = tera = T
1 000 000 000 = 109 = giga= G
1 000 000 = 106 = mega = M
1 000 = 103 = kilo = k
0.001 = 10-3 = milli = m
0.000 001 = 10-6 = micro = μ (pronounced “mu”)
0.000 000 001 = 10-9 = nano = n
0.000 000 000 001 = 10-12 = pico = p
Rules for Use of the metric prefixes
There are a few rules for the proper use of SI units
- Quantities that are defined by several units are separated by a dot “·” to avoid confusion with the prefix notations. For example a newton is correctly written as kg · m /s2 or kg · m · s-2 . Metre-second is written as m · s as opposed to millisecond which would be written as ms.
- When the unit is raised to an exponent, such as m2 or N2 , the prefix refers to both the unit and its prefix. This is very confusing. For volume, a cubic millimetre is written as mm3, it means (mm)3 = mm · mm · mm = 10-9 m3.
- The kilogram is an exception to this general rule. You want to keep kg as much as possible. In general, you want to avoid having the prefix in the denominator. Do not write seconds per mm or s/mm but write ks/m. Metres per milligram or m/mg should be written as Mm/kg as kg is a base unit.