abbreviations: 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.

The seven base units in SI are listed in the table below, with their symbols.

Derived units

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.

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

The table below gives a complete list of derived units.

Note: 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.

Prefixes

Multiples and submultiples of base units and derived units are expressed by adding one of the prefixes from the table below 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
• 4 dag
• 7 hL
• 13 kPa

Spacing

Leave a full space between the quantity and the symbol:

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

Hyphenation

For the sake of clarity, a hyphen may be inserted between a number and a unit name used adjectivally:

• 35-millimetre film
• 60-watt bulb

However, do not insert a hyphen if the unit symbol is used in place of the unit name:

• a 2 m length of fabric
• a 60 W bulb
• 35 mm film

Capitalization

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 CLOSER TO ALASKA

Exceptions

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

Use of symbol

SI usage prescribes that both number and unit name be written in full or that a numeral be used with a symbol:

• 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).

Additional units

Because of their practical importance, a number of additional units are approved for use with SI, although they do not, strictly speaking, form part of it.

Table 4

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)