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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.

Table 1

The seven base units with the International System of Units.
Quantity Unit name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

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.

Table 2

Complete list of derived units.
Name Symbol Quantity
coulomb C quantity of electricity, electric charge
degree Celsius °C Celsius temperature (see note)
farad F capacitance
gray Gy absorbed dose of ionizing radiation
henry H inductance
hertz Hz frequency
joule J energy, work, quantity of heat
lumen lm luminous flux
lux lx illuminance
newton N force
ohm electric resistance
pascal Pa pressure, stress
radian rad plane angle
siemens S electric conductance
sievert Sv dose equivalent of ionizing radiation
steradian sr solid angle
tesla T magnetic flux density
volt V electric potential, potential difference, electromotive force
watt W power, radiant flux
weber Wb magnetic flux

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.

Table 3

Lists of prefixes used with base and derived units.
Factor Prefix Symbol Factor Prefix Symbol
1024 yotta Y 10-1 deci d
1021 zetta Z 10-2 centi c
1018 exa E 10-3 milli m
1015 peta P 10-6 micro µ
1012 tera T 10-9 nano n
109 giga G 10-12 pico p
106 mega M 10-15 femto f
103 kilo k 10-18 atto a
102 hecto h 10-21 zepto z
101 deca da 10-24 yocto y

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:

  • cm
  • dag
  • 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:

  • a two-metre length of fabric
  • a sixty-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 orm

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

Additional units approved for use with SI
Quantity Unit name Symbol
time minute min
hour h
day d
year a
plane angle degree °
minute
second ’’
revolution r
area hectare ha
volume litre L
mass metric ton, tonne t
linear density tex tex

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)