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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:

Table 1

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

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.


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

Name Symbol Quantity
coulomb C quantity of electricity, electric charge
degree Celsius °C Celsius temperature3
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
  • 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:

Table 3

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, centimetres
  • dag
  • 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:



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 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:

  • cm3
  • 20 m2

Do not use abbreviations such as cc or cucm 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:

Table 4

Quantity Unit name Symbol
time minute min
hour h
day d
year a
plane angle degree °
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

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).