Units of measurement

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 (eg 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.

This section covers:

SI units
Non-SI units that can be used in conjunction with SI units
Other types of units.

Use of unit symbols with numbers (eg 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 unit^a	Symbol^a	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 unit^a	Symbol^a	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 (eg 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	Symbol^a	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.

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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 units^a

Unit	Symbol^a
minute	min
hour	h
day	d
degree (angle)	°
minute (angle)	'
second (angle)	"
litre	L
tonne (metric ton)	t
electron volt	eV
angstrom^b	Å
bar^b	bar
curie^b	Ci
hectare^b	ha
knot^b	knot
rad^b	rad
rem^b	rem
roentgen^b	R
nautical mile^b	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.

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	Symbol^a
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).

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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 (eg 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 (eg 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.

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Units of measurement

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Non-SI units

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