Style guide

Writing unit names and symbols, and expressing the values of quantities

The SI has rules and style conventions for the writing of unit names and symbols, including prefix names and symbols. Rules also exist for the expression of quantity symbols and quantities. Compliance with these rules aids understanding and readability, and reduces scope for confusion and misunderstanding.

Unit names

Unit names are printed in upright type, not in italic, bold or block capitals. They are treated like ordinary nouns. The names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalised material such as a title.
The newton is the unit of force.
The joule is the unit of energy.
The Newton is the unit of force.
The joule is the unit of energy.
In keeping with this rule, the correct spelling of the name of the unit with the symbol °C is “degree Celsius” (the unit degree begins with a lower-case d and the modifier Celsius begins with an upper-case C because it is a proper name).

Although the values of quantities are normally expressed using symbols for numbers and symbols for units, if for some reason the unit name is more appropriate than the unit symbol, the unit name should be spelled out in full.
15 MW
15 megawatts
15 Mwatts
15 megaW
When the name of a unit is combined with the name of a multiple or submultiple prefix, no space or hyphen is used between the prefix name and the unit name. The combination of prefix name and unit name is a single word.
milli metre
When the name of a derived unit is formed from the names of individual units, either a space or a hyphen is used to separate the names of the individual units.
newton metre

Unit symbols

Unit symbols are printed in upright type regardless of the type used in the surrounding text. They are printed in lower-case letters unless they are derived from a proper name, in which case the first letter is a capital letter. An exception is that for the litre, either capital L or lower-case l is allowed, in order to avoid possible confusion between the numeral 1 (one) and the lower-case letter l (el).
1 L of water has a mass of 1 kg.
A 1 kg mass weighs 9.8 N.
1 L of water has a mass of 1 kg.
A 1 Kg mass weighs 9.8 n.
A multiple or submultiple prefix, if used, is part of the unit and precedes the unit symbol without a separator. A prefix is never used in isolation and compound prefixes are never used.
The Moon is 384 Mm from the Earth.
The 10 km run is today.
One millionth of 1 kg equals 1 mg.
The Moon is 384 M m from the Earth.
The 10 K run is today.
One millionth of 1 kg equals 1 μkg.
Unit symbols are mathematical entities and not abbreviations. Therefore, they are not followed by a full stop except at the end of a sentence, and one must neither use the plural nor mix unit symbols and unit names within one expression, since names are not mathematical entities.
The bag contained 2 kg of apples.
The man is 180 cm tall.
Leeds is 273 km from London.
The bag contained 2 kgs of apples.
The man is 180 cm. tall.
Leeds is 273 kms from London.
In forming products and quotients of unit symbols the normal rules of algebraic multiplication or division apply. Multiplication must be indicated by a space or a half-high (centred) dot (⋅), since otherwise some prefixes could be misinterpreted as a unit symbol. Division is indicated by a horizontal line, by a solidus ( / ) or by a negative exponent. When several unit symbols are combined, care should be taken to avoid ambiguities, for example by using brackets or negative exponents. A solidus must not be used more than once in a given expression without brackets to remove ambiguities.
kW h
kg m-1 s-2
Abbreviations for unit symbols, or unit names, are not permitted. The use of the correct symbols for SI units, and for units in general, is mandatory.

Quantity symbols and unit symbols

Unit symbols must not be used to provide specific information about the quantity and should never be the sole source of information on the quantity. Units are never qualified by further information about the nature of the quantity; any extra information on the nature of the quantity should be attached to the quantity symbol and not to the unit symbol.
The water content is 20 mL/kg. Contents: 20 mL of water/kg.
Contents: 20 mLH2O/kg.

Formatting the value of a quantity

The numerical value always precedes the unit and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit. The space between the number and the unit is regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule are for the unit symbols for degree, minute and second for plane angle, °, ′ and ″, respectively, for which no space is left between the numerical value and the unit symbol. This rule means that the symbol °C for the degree Celsius is preceded by a space when expressing values of Celsius temperature t.
70 kg
21 °C
There are 360° in a circle.
There are 360 ° in a circle.
Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. Only when the name of the unit is spelled out would the ordinary rules of grammar apply, so that in English a hyphen would be used to separate the number from the unit.
a 3 m joist
a 20-metre hose
a 3-m joist
a 20 metre hose
In any expression, only one unit is used. An exception to this rule is in expressing the values of time and of plane angles using non-SI units. However, for plane angles it is generally preferable to divide the degree decimally. It is therefore preferable to write 22.20° rather than 22° 12′, except in fields such as navigation, cartography, astronomy, and in the measurement of very small angles.
The jump was 8.95 m in length.
The winner finished in 3 min 35 s.
The jump was 8 m 95 cm in length.

Expressing the value of a quantity in speech

The rule that only one unit is used when expressing the value of a quantity also applies when it is spoken.
e.g. 1.98 m when spoken:
“She cleared one point nine eight metres.” “She cleared one metre and ninety-eight centimetres.”
e.g. 65.07 m when spoken:
“The winning javelin throw was sixty-five point zero seven metres.” “The winning javelin throw was sixty-five metres and seven centimetres.”
e.g. 3.275 kg when spoken:
“The baby weighed three point two seven five kilograms.” “The baby weighed three kilograms and two hundred and seventy-five grams.”
e.g. 2.64 W when spoken:
“The power consumption was two point six four watts.” “The power consumption was two watts and sixty-four centiwatts.”

Formatting numbers, and the decimal marker

The symbol used to separate the integral part of a number from its decimal part is called the decimal marker. The decimal marker shall be either the point on the line or the comma on the line. The decimal marker chosen should be that which is customary in the language and context concerned.
If the number is between +1 and −1, then the decimal marker is always preceded by a zero.
For numbers with many digits, the digits may be divided into groups of three by a space, or half‑space, in order to facilitate reading. However, when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit.
Mount Everest is 8848 m high.
c = 299 792 458 m/s
c = 299792458 m/s
Mount Everest is 8 848 m high.
7.637 8
The practice of grouping digits in this way is a matter of choice; it is not always followed in certain specialised applications such as engineering drawings, financial statements and scripts to be read by a computer.

Neither dots nor commas are inserted in the spaces between groups of three digits.
c = 299 792 458 m/s
12 564.378 42
c = 299,792,458 m/s
c = 299.792.458 m/s
For numbers in a table, the format used should not vary within one column.
 radians degrees 
 1 57.295 779 5 
 2 114.591 559 0 
 3 171.887 338 5 
 radians degrees 
 1 57.2957795 
 2 114.591 559 0 
 3 171.8873385 

Mass vs weight

When the word “weight” is used, the intended meaning should be clear. In science and technology, weight is a force, for which the SI unit is the newton; in commerce and everyday use, weight is usually a synonym for mass, for which the SI unit is the kilogram.

Stating quantity values being pure numbers

Values of quantities with unit one, are expressed simply as numbers. The unit symbol 1 or unit name “one” are not explicitly shown. SI prefix symbols can neither be attached to the symbol 1 nor to the name “one”, therefore powers of 10 are used to express particularly large or small values.
4.7 × 10-9
10 000
2.3 × 1012
4.7 n
10 k
2.3 T
Quantities that are ratios of quantities of the same kind (for example length ratios and amount fractions) have the option of being expressed with units (m/m, mol/mol) to aid the understanding of the quantity being expressed and also allow the use of SI prefixes, if this is desirable (μm/m, nmol/mol). Quantities relating to counting do not have this option, they are just numbers.
10 mm/m
28.6 mmol/mol
The internationally recognised symbol % (percent) may be used with the SI. When it is used, a space separates the number and the symbol %. The symbol % should be used rather than the name “percent”. In written text, however, the symbol % generally takes the meaning of “parts per hundred”. Phrases such as “percentage by mass”, “percentage by volume”, or “percentage by amount of substance” shall not be used; the extra information on the quantity should instead be conveyed in the description and symbol for the quantity.
42 %
5.4 %
5.4 percent
The term “ppm”, meaning 106 relative value, or 1 part in 106, or parts per million, is also used. This is analogous to the meaning of percent as parts per hundred. The terms “parts per billion” and “parts per trillion” and their respective abbreviations “ppb” and “ppt”, are also used, but their meanings are language dependent. For this reason the abbreviations “ppb” and “ppt” should be avoided.
Pollutant concentration was 6.5 ppm Pollutant concentration was 6500 ppb