Units of Measurement

If someone told you they had a piece of rope 19 long, would it mean anything to you? Or if they said their friend weighed 100? How about if you were going to a party 6 away? As you can see from these examples, it is important to have previously defined "units" with which you can make measurements and how important it is to quote them to people when you want to communicate information about measurements.

For example, you could say the party is four times the distance to your mums house away, but that is a bit laborious and relies very heavily on shared experiences. If you were trying to explain to someone you never met (over the internet for example) then you would be stuck.

In the sciences, there is a commonly accepted method of passing this information on, and the units are known as "SI Units." This is an abbreviation for "Système Internationale d'Unités." The SI Units are maintained by a small agency located in Paris called "Bureau International des Poids et Mesures."

The background to the SI system can be summarised as "All systems of weights and measures, metric and non-metric, are linked through a network of international agreements supporting the International System of Units. The International System is called the SI, using the first two initials of its French name Système International d'Unités. The key agreement is the Treaty of the Meter (Convention du Mètre), signed in Paris on May 20, 1875. 48 nations have now signed this treaty, including all the major industrialized countries." (Rowlett, R., 2004). In 1960, an international conference finally, formally approved the system as a world wide scientific standard. As a result of the SI System, while in every day life you may drink a pint of milk or buy 5 pounds of potatoes in the shops, when it comes to science SI units are used almost exclusively.

The seven "base" units that form the foundations of the SI system are shown in table 1. From this further measurements are derived, some of which are shown in table 2. While the array of SI units may initially seem overwhelming, it is quite simple once you get used to the system. For most purposes the key ones to remember are:

  • SI unit for Length is the METRE (m)
  • SI unit for Time is the SECOND (s)
  • SI unit for Mass is the KILOGRAM (kg)

For the purposes of this web article, we will mainly deal with the base units in our examples.

Table 1 - SI Base Units
Property being measured SI Unit of Measure
Distance metre (m)
Mass kilogram (kg)
Time second (s)
Electric current ampere (a)
Temperature Kelvin (K)
Amount of a substance mole (mol)
Intensity of Light candela (cd)
Table 2 - SI Derived Units.
please note, this table doesnt show how these units are derived, this will be in a follow up article.
Property Derived unit
Frequency hertz (Hz)
Force newton (N)
Pressure pascal (Pa)
Energy (work) joule (J)
Power watt (W)
Electric charge coulomb(C)
Electric Potential volt (V)
Electric capcitance farad (F)
Electric resistance ohm (Ω)
Electric conductance siemens (S)
Magentic flux weber (Wb)
Magentic flux density tesla (T)
Inductance henry (H)
Temeperature degree Celsius (°C)
Plane angle radian (rad)
Solid angle steradian (sr)
Luminous flux lumen (lm)
Illuminance lux (lx)
Activity becquerel (Bq)
Absorbed dose gray (Gy)
Dose equvialent seivert (Sv)
Catalytic activity katal (kat)

Although these units, especially the base units, are helpful for measuring some objects, the metre is good for your walls for example, when the scale changes they can become a bit ridiculous and unwieldy. Fortunately the SI system has a process where we can discuss multiples of the numbers, so you dont have so say the distance between London and New York is 1,322,321m (for example, that was a made up number!).

Part of the SI system is adding prefixes onto the unit of measurement to indicate an "order of magnitude" change. From the base units we can see one already uses this prefix - kilogram. The word "kilo" at the start means this is actually 1,000 grams. In the same manner, you can say 1 kilometre instead of 1,000 metres. Conversely the prefix "milli" to mean one-thousandth (0.0001) so a millimetre is one thousandth of a meter. Table 3 outlines the units used for these order of magnitude changes, and you can learn more about them in the Scientific Notation article.


Rowlett, R., (2004) "Units: The International System," University of North Carolina at Chapel Hill [online] Available from http://www.unc.edu/~rowlett/units/sipm.html [Last visited 02 Jan 06]