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:
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:
Name | Symbol | Quantity |
---|---|---|
coulomb | C | quantity of electricity, electric charge |
degree Celsius | °C | Celsius temperature^{3} |
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 | Ω | 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 |
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:
Factor | Prefix | Symbol | Factor | Prefix | Symbol |
---|---|---|---|---|---|
10^{24} | yotta | Y | 10^{-1} | deci | d |
10^{21} | zetta | Z | 10^{-2} | centi | c |
10^{18} | exa | E | 10^{-3} | milli | m |
10^{15} | peta | P | 10^{-6} | micro | µ |
10^{12} | tera | T | 10^{-9} | nano | n |
10^{9} | giga | G | 10^{-12} | pico | p |
10^{6} | mega | M | 10^{-15} | femto | f |
10^{3} | kilo | k | 10^{-18} | atto | a |
10^{2} | hecto | h | 10^{-21} | zepto | z |
10^{1} | deca | da | 10^{-24} | yocto | y |
The prefix and unit name are always spelled as one word:
When symbols are used, the prefix symbol and unit symbol are run together:
Leave a full space between the quantity and the symbol:
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):
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:
Current usage, however, accepts the use of numerals with spelled-out unit names to facilitate comprehension:
In scientific and technical writing, the preferred form is numerals with unit symbols:
When no specific figure is stated, write the unit name in full:
Area and volume in the metric system are expressed by means of superscript numerals:
Do not use abbreviations such as cc or cu. cm for cubic centimetre (cm^{3}), 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:
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:
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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