[1.144] Symbols

1.144 Symbols

Always define the units of measure that are used in the document, and their abbreviations and symbols, at the start of the text. Symbols that are widely known (distance, mass, volume, and time—see Standard abbreviations for symbols in common use that do usually not require definition). Repeat these definitions as often as you think is necessary to avoid any possibility of misunderstanding. Even the most widely used symbols, prefixes, and abbreviations are frequently used incorrectly. For example, what is the correct symbol for the kilotonne: kt, Kt, kT or KT?

The shortened form of an SI unit of measure is known as a symbol, and obeys different punctuation rules to abbreviations. Non-SI units can, however, use abbreviations. There are many exceptions and should be studied carefully.

1.145 Symbols, usage

The following is a summary of the more important rules for use of symbols.

•	The plural of a symbol is the same as the singular—for example, 1 cm and 2 cm.

•	When units of measure are not abbreviated, use the singular for quantities of one or less, except with zero, which takes the plural (0 inches).

•	Put a non-breaking space between the expression of measurement and the associated unit, except for percent (%) and planar angle (° ' " ), which are the only symbols that follow the measurement without a space.

•	Symbols are not abbreviations and do not have periods, except the symbol for inch, which always takes a period.

•	If the symbol is well-known (°C, %, m, and kg, for example), it can be used without definition (see Units of measure, common symbols). Otherwise, define your symbols at first use and in the list of abbreviations in the front matter.

•

Always use the spelled out form when discussing units of measure (“The results are shown in megawatt hours.”). In general, it’s okay to use symbols when stating a measurement (“The pipeline has a capacity of 35 TJ/d”), particularly if measurements occur frequently, and if you have space constraints—in a table for instance.

For a more complete description, refer to the following topics:

•	Metric system

•	Imperial system

[1.146] Metric system

For guidelines on using SI symbols and when they should be spelled out, see Symbols, usage.

1.146 SI symbols

In the International System of Units (SI), the units do not have “abbreviations”. They have symbols. The unit symbols do not follow the grammatical rules for abbreviations, because they follow the mathematical rules for symbols instead. These rules include the following.

Note. The format and punctuation of SI symbols is an ISO standard, which is applied in Australia through AS ISO 1000–1998.

1.147 Punctuation, symbols

Symbols are always separated from the numerical quantity they follow by a non-breaking space, except for percent (%) and planar angle (° ' "),  which are the only symbols that follow the measurement without a space. Symbols can be preceded by a prefix, as in GB and kW, but must never be placed next to another symbol.

A symbol is never followed by a period, except the imperial symbol for inch, which always has a period. If the symbol is at the end of a sentence, normal punctuation is applied.

1.148 Plurals, symbols

The letter “s” is never added to a symbol to indicate a plural.

Note. The name of the unit is a noun, is not capitalized (except degree Celsius), and obeys the standard rules of punctuation when spelled out.

Incorrect

5in.
2cm
2 cms
7m
5 Kg
1.2 cts
38 %
15 ° 45 ' 10 "
38.6°C

Correct

5 in.
2 cm
7 m
5 kg
1.2 ct
38%
15° 45' 10"
38.6 °C

1.149 Case, prefixes

Symbols must be written as they are defined no matter where they occur in a sentence or title. Capitalizing a symbol changes its meaning. For example, t is the symbol for the tonne, and T is the symbol for the tesla and the tera prefix.

There is a tradition in the metric system that the first (or only) letter of an unprefixed unit symbol is capitalized if (and only if) the unit's name comes from a proper name. Thus W is the symbol for the watt, and A is the symbol for the ampere, because these units are named for scientists. The symbol for litre is an exception, and is normally capitalized. Both L and l are acceptable but L is preferred, because the letter l is easily confused with the number 1. Note that the spelled-out unit is not capitalized (joule, for example) except for degree Celsius and degree Fahrenheit (but you must include the uncapitalized “degree”).

The case of symbol prefixes is specified, upper and lower, and must not be changed. For example, the symbol for kilo- is k-, so kW and not KW is the symbol for the kilowatt. And the correct symbol for kilotonne is kt, and never Kt or kT.

The symbol for kilogram kg is the only SI symbol that includes a prefix. Only one prefix can be used on a symbol. Hence km is the symbol for (kilo-)metre, but mkg (milli-)(kilo-)gram is not permitted.

There are twenty SI prefixes. The most widely used prefixes are:

Symbol	Name	Factor
n	nano-	10-9
µ	micro-	10-6
m	milli-	10-3
k	kilo-	103
M	mega-	106
G	giga-	109
T	tera-	1012
P	peta-	1015
E	exa-	1018
Z	zeta-	1021

Note that the prefix k, although an anomaly (prefixes that signify a factor of greater than 1 are otherwise upper case), is never capitalized, as in kB, km, kg, kPa, and kL.

1.150 Derived units (exponent, factor, per)

The superscripts 2 and 3 are always used for “square” and “cubic”, respectively.

A middle dot · is used to separate multiplied symbols, as in kilowatt hour kW·h —symbols must not be placed next to each other.

Note. In cases where a non-standard format is in general use and does not cause confusion—kWh, for example—define it as an abbreviation. For more information, see Non-standard units.

The slash mark / is used for “per”. Furthermore, only one slash mark is allowed per symbol.

Correct

km2
kW·h
m/s2 (engineering)
m·s-2 (scientific)

Incorrect

km2
sq km
kWh
kW·hr
m/s/s

Note. You will find instructions on how to type a middle dot, non-breaking space, thin space, and other special characters in Typing shortcuts.

1.151 Non-standard units

Although the correct SI format is always preferred, in cases where a non-standard format is widely accepted in an industry, it can be defined as an abbreviation and then used legitimately. This list also includes measurements for which there are no accepted SI symbols, such as machine-hour and bank cubic metre.

 

Symbol	Usage
BCM, LCM, kBCM	The terms bank cubic metre (in situ volume of rock in m3) and loose cubic metre (broken volume of rock in m3) are widely used in the mining and construction industries. They are also useful when expressing large volumes (1000 m3 = kBCM, not km3). The SI-compliant km3 (= GBCM) is rarely used.
kWh	Although the compliant kW·h is strongly recommended, if the non-compliant kWh is used, it must be defined as an abbreviation on first use. Add this to your list of abbreviations: “[abbreviation]kWh [term]kilowatt hour, also kW·h”.
kPaG	The non-compliant kPaG (kilopascal gauge) should always be written as kPa gauge, unless space is a consideration. Always define as an abbreviation on first use. Also kPaA, although the A (absolute) is implied and best omitted.
m-hr, t/m-hr, m/m-hr, $/m-hr	Units based on machine-hours (m-hr) are used widely in mining to measure productivity (metres per, tonnes per, dollars per). Also l-hr (labour-hour). Define on first use and do not use manhour.
Moz, koz, Mlb, kton, Mton	Moz (million ounces), koz (thousand ounces), Mlb (million pounds), kton (thousand tons), and Mton (million tons) incorrectly mix SI prefixes with an imperial unit, but they are widely used in the mining industry. Moz and koz are often used to define reserves of gold, and Mlb is sometimes applied to uranium and copper reserves or stocks.
MT, DMT, MDMT, MDMTPA	Avoid using these terms if possible. They are used in shipping of bulk commodities (like iron ore and wheat). Use of these terms is both non-standard and variable. A metric ton (MT) is a tonne (t) and is sometimes (incorrectly) written “metric tonne”. If there is any chance of confusion between ton (ton) and tonne (t), write “(metric) tonne” or (US) short ton or (UK) long ton. These abbreviations are capitalized to prevent confusion with the SI symbols t and Mt (million tonnes). If the lower-case dmt is used in your sources of information, use your judgement on whether to follow their style (for consistency) but ensure that there is no possibility of confusion. Avoid adding SI prefixes to MT and DMT: the conflicting MT (million tons or megatesla or metric ton) should tell you why. However, MDMT (million dry metric tons) and MDMTPA (million dry metric tons per annum) are common. The preferred forms are Mt (dry) and Mt/yr (dry) or Mtpa (dry). Also see entry for ton in List of SI symbols.
Mtpa, tpd, tph	The term Mtpa (million tonnes per annum) is widely used in the mining industry and better known than the compliant Mt/yr. Also tpd and tph are often used to describe production rates of equipment and are more common than the compliant t/d and t/h.
MVAr	The abbreviation for volt-ampere reactive is VAr. The abbreviation MVAr is pronounced “megavar”. Always define as an abbreviation on first use.
scm, kscm	The non-compliant scm (standard cubic metre) is widely used in the energy sector as a standard measure of gas volume (the volume at 1 atmosphere of pressure and 15 °C [288 K]). Always define as an abbreviation on first use.

[1.152] List of SI symbols

1.152 List of SI symbols

The base, derived, and acceptable SI units (International Organization for Standardization) and their symbols are identified in the following table. Other units shown are outside the SI system but are widely used. For simplicity, treat units that are outside the SI system as symbols (see also Imperial system, below).

Refer to Glossaries for symbols and abbreviations that are specific to an industry.

 

Symbol Name	Symbol	SI	Notes
ampere	A	SI	Base SI unit of electric current. Commonly known as the amp. In technical documents, always refer to as ampere. Also kA, mA, and µA.
bar	bar	SI	Acceptable unit of pressure.
baud	baud		Data transmission speed.
bit	bit		Data size (also kbit, Mbit, Gbit).
byte	B		Data word size (also kB, MB, GB). Note that Microsoft has adopted KB but should be ignored.
carat	ct		Unit of mass equal to 200 mg (gems).
centimetre	cm	SI	Derived SI unit of length. Not centimeter (US).
coulomb	C [A·s]	SI	Derived SI unit of electrical charge. For storage batteries use ampere hour (1 A·h = 3.6 kC).
cubic feet	cu. ft.		Abbreviation.
cubic metre	m3	SI	Derived SI unit of volume.
day	d	SI	Acceptable.
degree Celsius	°C	SI	Derived SI unit of temperature. Not centigrade.
degree Fahrenheit	°F		Imperial.
degree	°	SI	Acceptable unit of planar angle.
dots per inch	dpi		Typography unit of printing density.
farad	F [C/V]	SI	Derived unit of electrical capacitance (also pF, nF, µF).
feet	ft		Imperial (also foot, ' ).
gallon	gal		Imperial.
gram	g [kg/1000]	SI	Derived SI unit of mass.
grams per litre	g/L	SI
grams per tonne	g/t	SI
hectare	ha	SI	Acceptable.
henry	H [V·s/A]	SI	Derived SI unit of electrical inductance (also µH, mH).
hertz	Hz [s-1]	SI	Derived SI unit of frequency (also kHz, MHz, GHz).
hour	h	SI	Acceptable. Use for periods of time. For clock time, use the abbreviation hr or hrs.
inch	in.		Imperial (also inches, " ); always with a period.
joule	J [N·m]	SI	Derived SI unit of energy (also kJ, MJ, GJ, TJ). Equals one watt second.
karat	K		Unit of purity (gold), as in 24K.
kelvin	K	SI	Base SI unit of temperature. Always kelvin or K, never Kelvin, °K, or degree kelvin.
kilogram	kg	SI	Base SI unit of mass.
kilograms per cubic metre	kg/m3	SI	Derived SI unit of volumetric density.
kilohertz	kHz	SI
kilometre	km	SI	Not kilometer (US).
kilometre tonne	km·t		Also tonne kilometre (mining)
kilometres per hour	km/h	SI	Derived SI unit of velocity (automotive).
kilopascal gauge kilopascal absolute	kPa gauge kPa absolute	SI	Do not use kPaG or kPaA. The descriptor “absolute” is implied and can be omitted.
kilowatt hour	kW·h	SI	Derived SI unit of electrical energy (also MW·h). One kilowatt hour equals 3.6 megajoules. MWh and kWh are abbreviations and must always be defined if used.
knot	knot		nautical mile per hour
litre	L	SI	Lower-case l is permitted but not recommended, always mL for millilitre (also kL, ML).
machine-hour	m-hr		See Non-standard units.
metre	m	SI	Base SI unit of length. Not meter.
metres per second	m/s	SI	Derived SI unit of velocity.
metres per second squared	m/s2	SI	Derived SI unit of acceleration.
micrometre	µm	SI	Replaced micron (obs.).
micron	µ		Obsolete. See micrometre.
microsecond	µs	SI
mile	mi
miles per hour	mph
millilitre	mL	SI	Not milliliter, mil, ml, or ML (megalitre).
millimetre	mm	SI	Not millimeter (US).
millimicron	mµ		Obsolete. See nanometre.
million years	Ma		Geological term.
millisecond	ms	SI
minute	'		Angle unit equal to 1⁄60 of a degree.
minute	min	SI	Time. Acceptable for use with SI.
month	mo., mos		Abbreviation; always spell out.
nanometre	nm	SI	Replaced millimicron (obs.).
nanosecond	ns	SI
nautical mile	nautical mile
newton	N [m·kg·s-2]	SI	Derived SI unit of force (also kN, MN).
ohm	Ω [V/A]	SI	Derived SI unit of electrical resistance (also mΩ, kΩ, MΩ).
ounce (avoirdupois)	oz		Imperial, standard US measure. 1 ounce (avdp) = 28.35 grams
ounce (troy)	oz		Imperial, precious metals. 1 ounce (troy) = 31.10 grams
ounces per ton	oz/ton		Imperial, mining. Usually means troy ounces (of gold) per short ton (of rock). Converting to SI, 1 oz troy per short ton equals 31.10 g per 907.18 kg, which equals 34.28 ppb or 34.28 g/t.
parts per million	ppm		Also ppb (per billion). Beware of ppt, which can mean either parts per thousand or per trillion.
pascal	Pa [N·m2]	SI	Derived SI unit of pressure (also kPa, MPa, GPa).
pennyweight	dwt		Obsolete.
percent	%		Measurement of concentration (1⁄100), usually volumetric or population.
pica	pi		Typography unit equal to 1⁄72 of a foot.
pixel	px		Screen resolution.
point	pt		Font size.
pound	lb		Imperial, avoirdupois or standard pound. 1 lb = 16 oz (avdp). Also troy pound ( = 12 oz troy) but rare.
radian	rad	SI	Derived SI unit of planar angle.
second	s	SI	Base SI unit of time. Not sec (abbrev.).
second	"		Angle unit equal to 1⁄60 of a minute.
square metre	m2	SI	Derived SI unit of area. Not sqm or m2.
tesla	T [V·s/m2]	SI	Derived SI unit of magnetic flux density (also µT, mT).
thousandth of an inch	mil		Imperial.
ton	ton		Imperial. Do not abbreviate. Do not confuse with t (tonne) or T (tesla). Usually (US) short ton, but ambiguous: (US) short ton (2000 lb = 907.18 kg), (UK) long ton (2240 lb = 1016.05 kg), or (US) metric ton (2204.62 lb = 1000 kg), where lb is avoirdupois (standard) pound. To avoid confusion with t (tonne) and Mt (million tonnes), capitalize the abbreviations ST (= ton), LT, MT (= t), DMT (dry metric ton). Avoid adding SI prefixes, but kton (thousand short tons) and Mton (million short tons) are common.
tonne	t		Also known as a “metric ton” (US). Standard unit of mass equal to 1000 kg. Also see ton.
tonnes per annum	tpa		Abbreviation (mining). Do not use t/yr (SI) or t/a. Also Mtpa, tpd, and tph.
volt	V [W/A]	SI	Derived SI unit of electrical potential or electromotive force (also µV, mV, kV). Avoid the term “voltage”.
volt ampere	V·A	SI	Measurement of active or apparent electrical power. VA is an abbreviation, not an SI symbol. Also see VAr.
volt ampere reactive	VAr		Abbreviation. Measurement of reactive power. Also MVAr (pronounced “megavar”).
watt	W [J/s]	SI	Derived SI unit of power (also kW, MW, GW).
week	wk		Abbreviation; always spell out.
year	yr		Abbreviation; always spell out. The expression per annum and its abbreviation pa are acceptable—use the style that is most familiar to your audience. But do not use the symbol a in place of yr, as in t/a. See also million years (Ma).

[1.153] Imperial system

1.153 Imperial system

Imperial or traditional English units are less regular than SI units and are prone to error and misinterpretation. A variety of abbreviations are used. Sometimes these abbreviations duplicate metric symbols; for example, A is sometimes used in English for the acre instead of the ampere. It would be better to use the symbol ac for the acre.

The recommended practice is to define symbols for the traditional English units and to apply the SI rules to their use. This is done for both consistency and clarity. It avoids a number of outstanding problems caused by the traditional abbreviations for the English units. For example, the abbreviation psi is commonly used for pounds per square inch, however the correct unit of measure is pounds force per square inch, which has the algebraic symbol lbf/in.2. In this case, using the more recognizable symbol psi, defined in the text as pounds force per square inch (lbf/in.2), avoids any confusion. A more general example is the symbol for pound (lb).

In general, do not add an SI prefix to an imperial unit (see Non-standard units for exceptions). The SI symbol formatting rules otherwise apply:

Correct

2 lb
2 pounds
1 pound
2 million tons

Incorrect

2lb
2 lb.
2 lbs
2 pound
2 Mton