The International System of Units (Système International d’Unites; the SI system) is used in Australia and most other countries to express metric measurements for quantities. The SI system replaced the former imperial system in Australia in 1970. The imperial system (e.g. inch/foot/yard/mile; ounce/pound/stone/ton; fluid ounce/pint/quart/gallon) continues to be used in the United States and to some extent in the United Kingdom.
In Australia, the imperial system persists in some measurements, including tennis net height (3 feet, or 1 yard), cricket pitch length (22 yards), screwdriver calibration (turns to the inch), print dimensions (picas, which are one-sixth of an inch), altitude of jet planes (feet) and nautical miles.
Use of unit symbols with numbers (e.g. spaces between numbers and units) is addressed in Numbers used with units.
SI units
The SI system has 7 base units (see the table ‘SI base units and their symbols’), from which others have been derived (see the table ‘SI derived units that have been given special names’).
SI base units and their symbols
| SI unita | Symbola | Quantity |
|---|---|---|
| ampere | A | electric current |
| candela | cd | luminous intensity |
| kelvin | K | thermodynamic temperature |
| kilogram | kg | mass |
| metre | m | length |
| mole | mol | amount of substance |
| second | s | time |
a Shows correct capitalisation
SI derived units that have been given special names
| SI derived unita | Symbola | Derived quantity |
|---|---|---|
| becquerel | Bq | activity (radionuclide) |
| coulomb | C | electric charge, quantity of electricity |
| degree Celsius | °C | Celsius temperature |
| farad | F | capacitance |
| gray | Gy | absorbed dose, specific energy imparted |
| henry | H | inductance |
| hertz | Hz | frequency |
| joule | J | energy, work, quantity of heat |
| katal | kat | catalytic activity |
| 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 |
| volt | V | electric potential difference |
| watt | W | power, radiant flux |
| weber | Wb | magnetic flux |
a Shows correct capitalisation
Many other derived units have not been given special names (e.g. square metre [m2], amperes per metre [A/m], metres per second [m/s]).
When written out in full, units with a proper noun do not have an initial capital. However, the symbols for these units do have an initial capital:
pascal [symbol Pa] newton [symbol N] watt [symbol W]
Caution! Exception: In the unit ‘degrees Celsius’, the capital C is maintained.
SI units can be used with standard prefixes to show larger or smaller quantities. Standard prefixes and their values are shown in the table below. Take great care to use the correct lower-case or capital letter for the symbol representing a prefix.
Beware the transfer of the μ (micro) symbol (character code 181 in Microsoft Word) between computer programs, to avoid inadvertent conversion to m.
Standard SI prefixes
| Prefix | Symbola | Value |
|---|---|---|
| yotta | Y | 1024 |
| zetta | Z | 1021 |
| exa | E | 1018 |
| peta | P | 1015 |
| tera | T | 1012 |
| giga | G | 109 |
| mega | M | 106 |
| kilo | k | 103 |
| hecto | h | 102 |
| deca | da | 101 |
| deci | d | 10–1 |
| centi | c | 10–2 |
| milli | m | 10–3 |
| micro | µ | 10–6 |
| nano | n | 10–9 |
| pico | p | 10–12 |
| femto | f | 10–15 |
| atto | a | 10–18 |
| zepto | z | 10–21 |
| yocto | y | 10–24 |
a Shows correct capitalisation
Caution! Be careful to check that the μ symbol used in units is not accidentally converted to m between computer programs.
Non-SI units
Some other units (which are not part of the SI system) can be used in conjunction with SI units; common ones are listed in the table below. They include some units that are widely used in science.
Common units accepted for use with SI unitsa
| Unit | Symbola |
|---|---|
| minute | min |
| hour | h |
| day | d |
| degree (angle) | ° |
| minute (angle) | ' |
| second (angle) | " |
| litre | L |
| tonne (metric ton) | t |
| electron volt | eV |
| angstromb | Å |
| barb | bar |
| curieb | Ci |
| hectareb | ha |
| knotb | knot |
| radb | rad |
| remb | rem |
| roentgenb | R |
| nautical mileb | n mile |
a Only the more common units are listed here; for a more detailed listing, including further details on when it is appropriate to use these symbols, see the Council of Science Editors Scientific style and format(Opens in a new tab/window).
b These units are permitted but not encouraged within the SI system.
Other commonly used non-SI units and symbols are listed in the table below.
Other commonly used units
| Unit | Symbola |
|---|---|
| acidity or alkalinity of an aqueous solution | pH |
| molarity | M |
| molality | m |
| normality | N |
| percentage | % |
| dollar | $ (see Currency) |
a Shows correct capitalisation and italics
Some other units, not recognised within the SI system, are legally recognised in Australia. They include decibel (dB), metric carat (CM or ct), stokes (St) and poise (P).
Other types of units
Some units define the activity of a chemical, drug or biological agent:
ED = effective dose
ID = infective dose
LD = lethal dose
LC = lethal concentration
A number immediately after the abbreviation indicates the level of activity:
ED50 = 50% effective dose [dose that is effective for 50% of samples]
Such numbers used to be set as subscripts (e.g. LD50), but there is now a trend to set them inline. See also Pharmacokinetics and toxicokinetics.
Define these units at the first use in the text and in tables, as appropriate.
Other units are derived from terms referring to the behaviour of a biological system, or performance of an instrument. Some of these are abbreviated to lower-case initialisms, while others take capitals:
CFU = colony forming unit
PFU = plaque forming unit
cpm = counts per minute
IU = international unit
rpm = revolutions per minute
g [italic; not G] = acceleration due to gravity near Earth's surface [not ‘gravitational force’]
As with other units, do not add a plural s to these abbreviations:
10 CFU not 10 CFUs
Apart from very familiar ones (e.g. rpm), these should be defined the first time they are used in the text and in tables.
Acceleration due to gravity near Earth's surface (g) is often used as a relative unit for acceleration. An acceleration of 3 times that due to gravity can be referred to as a ‘3 g acceleration’. When specifying performance of laboratory centrifuges, g is a more useful measure than rpm because rpm does not take into account the radius of the centrifuge arm.