1.23 The International System of Units (SI)

The International System of Units (SI), which has replaced other metric systems and is now used in Canada and many other countries, is a decimal-based system that includes units for physical quantities.

There are seven base units in SI:

In addition, a number of derived units are used. Like the kelvin and the ampere, almost all of them are named after scientists associated with a scientific discovery. Thus, when the symbol is used, its initial letter is capitalized. When written in full, however, the unit name is in lower case, e.g. H for henry and F for farad.

The International System of Units, Exception

Celsius takes an initial capital whether written in full or as a symbol.

The table below gives a complete list of derived units:

• Back to the note³ The Celsius temperature scale (previously called Centigrade, but renamed in 1948 to avoid confusion with "centigrad," associated with the centesimal system of angular measurement), is the commonly used scale, except for certain scientific and technological purposes where the thermodynamic temperature scale is preferred. Note the use of upper-case C for Celsius.

Multiples and submultiples of base units and derived units are expressed by adding one of the prefixes from the following table directly to the unit name:

The prefix and unit name are always spelled as one word:

• centimetre
• decagram
• hectolitre
• kilopascal

When symbols are used, the prefix symbol and unit symbol are run together:

• 5 cm, centimetres
• 4 dag
• 7 hL
• 13 kPa

Leave a full space between the quantity and the symbol:

• 45 kg, kilograms (not 45kg, kilograms)
• 32 °C (not 32°C)

For the sake of clarity, a hyphen may be inserted between a numeral and a symbol used adjectivally (see also 2.10 Numerals and units of measurement):

• 35-mm film
• 60-W bulb

Unit symbols and prefixes should always be in lower case, even when the rest of the text is in upper case:

• SIBERIA DRIFTS 5 cm, centimetres CLOSER TO ALASKA

The International System of Units, Exceptions

The symbol L for litre (to distinguish it from the numeral 1) and, as mentioned above, those symbols derived from the names of scientists.

SI usage prescribes that both numeral and unit name be written in full or that both be abbreviated:

• two metres or 2 m

Current usage, however, accepts the use of numerals with spelled-out unit names to facilitate comprehension:

• He ran the 100 metres in 10 seconds.

In scientific and technical writing, the preferred form is numerals with unit symbols:

• The specific latent heat of fusion of sulphur is 38.1 K/kg.

When no specific figure is stated, write the unit name in full:

• The means of transportation chosen depends on how many kilometres an employee has to travel to work.

Area and volume in the metric system are expressed by means of superscript numerals:

• 5 cm³
• 20 m²

Do not use abbreviations such as cc or cu. cm for cubic centimetre (cm³), kilo for kilogram (kg), amp for ampere (A) or kph for kilometres per hour (km/h).

Because of their practical importance, the following additional units are approved for use with SI, although they do not, strictly speaking, form part of it:

Note that there is no standard symbol for week or month. These units should therefore always be spelled out in technical writing.

When a unit symbol is combined with a symbol for time, or with a derived unit implying a division, an oblique (/) separates the two:

• 80 km/h
  • not 80 kmh or 80 kph
• 1800 r/min
  • not 1800 rpm
• 50 A/m
  • not 50 Am
• 200 J/kg
  • not 200 Jkg

More detailed information on the International System of Units (SI) can be found in the National Standard of Canada, Canadian Metric Practice Guide (CAN/CSA-Z234.1-89).

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