Writing Tips

SI Units (Metric system) (1)

This tip is the first of several about expressing the units of the Système international d'unités—also known as SI, or (incorrectly) the metric system. This tip introduces the common base units and prefixes, and the special base units.

The New Globalization
Civilizations have always looked for ways to measure physical objects: their size, their lightness or heaviness, their capacity to hold other things. Many cultures created entire systems to measure basic physical properties: length, area, volume, and weight. Owing to the spread of the British Empire, the Imperial system of measures (avoirdupois) became the system of choice in many countries (an early expression of globalization!). Although cultural systems of measurement persist in many places, international commerce has caused the world to adopt SI measures.

Why SI?
The basic idea behind SI is to accept a base unit for each property being measured, then to add prefixes that multiply the base unit by powers of 10 to obtain larger and smaller units of the given property. Because Arabic numbering (the world-wide standard) is based on powers of 10, SI is considered more scientific and rational, and easier to learn and manipulate than systems that use more eccentric unit labels. For example, compare:

• Imperial: 12 inches to the foot; 3 feet to the yard; 5.5 yards to the rod; 320 rods to the mile
• SI: 10 centimetres to the decimetre; 10 decimetres to the metre; 1000 metres to the kilometre (passing the decametres and the hectometres on the way)

In the Imperial system, nothing signals that feet, yards, rods, and miles are all length measurements. You must memorize the units and their relationships. On the other hand, the base unit of length in SI is the metre, and as soon as you see a unit word containing "metre", you know that you are measuring length. The prefix (centi, deci, kilo) tells you how large a length unit you are dealing with.

Base Units
Here are the base units in SI for the most common physical properties:

• Length: metre

Note the spelling. In Canada and Britain, a metre is a measurement of length; a meter is a measuring device ("thermometer"). In the United States, meter is used in both cases.

• Area: square metre
   
• Volume: cubic metre
   
• Mass: gram

Notice that the measurement of an object's inertia is technically called mass in SI. Weight is an expression of the pull of gravity, which may differ from one place in the universe to another. In non-technical contexts, mass is not likely to displace weight ("I think I think I need to reduce my mass"). However, mass is preferred in contexts where technical precision is important ("The mass of this moon rock is unusual").

Prefixes and Derived Units
By adding the prefixes listed below to a base unit, you obtain derived units that are smaller or larger (by powers of 10) than the base unit. These are the more common prefixes:

Therefore, a centimetre is 0.01 metre; a square kilometre is 1,000,000 square metres; a cubic decimetre is 0.001 of a cubic metre; and a decagram is 10 grams.

Special Units
While the SI base units and prefixes can be used to express any measurement, some additional "convenience" units have been accepted because of their usefulness in daily transactions:

• Area: are (= 100 square metres)

The are is most commonly seen with the prefix hecto: a hectare (10,000 square metres) is the common unit used to describe small expanses of land (for example, farms, parks, and rural lots). Large land expanses are typically measured in square kilometres.


• Volume: litre (= 1,000 cubic centimetres)
   
• Mass: tonne (= 1,000 kilograms)

The tonne, being close in mass to the 2240 pounds of an avoirdupois (long) ton, is also sometimes called a "metric ton". Tonne, however, is the preferred term.