SI Units Rules and Style Conventions

To maintain consistency in SI units, the International System of Units has adopted specific rules and style conventions for writing SI units. General rules for writing SI units were first specified by the 9^th CGPM in 1948. Later, various international bodies like ISO extended these rules. These rules are very important in scientific writing. Adherence to these rules is strictly recommended while submitting and reviewing a manuscript. The general guidelines are described below.

General rules

Rule 1

Mixing of SI units with other units should be avoided. The formation and usage of unusual units are unaccepted. Consider the below examples.

Correct: Surface tension of water is 0.072 N m⁻¹ at SATP.

Incorrect: Surface tension of water is 0.021 9 N ft⁻¹ at SATP.

Correct: The diameter of the hydrogen atom is 120 × 10⁻¹² m.

Incorrect: The diameter of the hydrogen atom is 393.70 × 10⁻¹² ft.

Rule 2

Non-standard symbols and notations are not recommended. Only the standard symbols and notations are used for prefixes and units.

Correct: m³, cubic metre; m s⁻¹, metre per second; N m, newton-metre,

Incorrect: cc, cubic centimetre; mph, miles per hour; lb_f in, inch pound-force.

Rule 3

SI symbols with a plural number remain unchanged. But when the full form of symbols is used, we must treat them as an English word.

Correct: I run 2 km a day. I run 2 kilometres a day.

Incorrect: I run 2 kms a day. I run 2 kilometre a day.

Rule 4

SI units do not end up with a full stop except at the end of a sentence. We do not have to add any punctuation (full stop, comma, semicolon) to SI units unless grammar of a language demands it.

Correct: You will weigh 10 kg on the moon.

Incorrect: “You will weigh 10 kg. on the moon” or “You will weigh 10 kg, on the moon”

Rule 5

The multiplication between two SI units is indicated through a space or an interpunct (a middle dot). And division is represented by a slash (a solidus) or a negative power. The repetition of slashes is avoided.

Correct: J/(K mol), J/(K·mol), J·K⁻¹·mol⁻¹, J K⁻¹ mol⁻¹

Incorrect: J/mol/K, J/(Kmol), JK⁻¹mol⁻¹

Rule 6

Prefixes are not separated from their units by any punctuation mark or space. They are considered as one unit. Also, Compounding a prefix is not acceptable.

Correct: The diameter of a particle is 6 nm.

Incorrect: The diameter of a particle is 6 n m, the diameter of a particle is 6 µµm.

Rule 7

Alteration of the case of a prefix can change the value of an SI unit. Consider 100 MW of electricity and 100 mW of electricity. The former means 10⁸ W of electricity while the later means 0.1 W of electricity. Such mistakes should be avoided.

Rule 8

SI units and SI symbols cannot be mixed.

Correct: m³/kg, cubic metre per kilogram

Incorrect: m³ per kilogram, cubic metre per kg

Rule 9

If a unit is named after the name of a person, the first letter of the unit’s symbol should be always in the capital.

Correct: K, kelvin; Pa, pascal; T, tesla

Incorrect: k, kelvin, pa, pascal; t, tesla

Rule 10

Certain symbols of units like L of litre can be written in small as well as in the capital. The NIST recommends capital L for litre.

Rule 11

Variable parameters should be represented in italic while constant parameters, SI units, prefixes, numbers should be in roman.

Correct: Heat capacity at constant pressure is denoted as c_p. The heat capacity of water is 4 185 J/(kg·K).

Incorrect: Heat capacity at constant pressure is denoted as c_p. The heat capacity of water is 4 185 J/(kg·K).

Correct: The pressure of the atmosphere is denoted as P_atm.

Incorrect: The pressure of the atmosphere is denoted as P_atm.

Rule 12

We cannot modify SI units by attaching any descriptive information to them. The mixing of descriptive information with SI symbols is not allowed.

Correct: 10 kg of water

Incorrect: 10 kg^water, 10 kg_water

Correct: SO₂ concentration is 200 µg m⁻³ in the sample.

Incorrect: 200 µg SO₂ m⁻³ in the sample.

Rule 13

The value of a quantity and its respective unit is separated by space. This is true for all the units including % and ℃. The angular symbols (degree, minute, second) are exceptions. As per general English writing, we usually do not add space between a number and % symbol. But according to the SI brochure and ISO, we need to add space between them.

Correct: 10 m² s⁻¹, 14 %, 30 ℃, 4°, 20′, 32″

Incorrect: 10m² s⁻¹, 14%, 30℃, 4 °, 20 ′, 32 ″

Rule 14

When arithmetic operators are used, the proper placement of SI units is necessary to avoid any ambiguity.

Correct: 150 cm ± 20 cm

Incorrect: 150 ± 20 cm

Correct: 4 m × 4 m, (4 × 4) m

Incorrect: 4 × 4 m

Correct: 5 ℃ to 20 ℃, (5 to 20) ℃

Incorrect: 5 to 20 ℃

Rule 15

When a number and its unit in full form is used in a sentence as an adjective, we treat them as normal English words.

Correct: You have to gain 15-kilogram weight.

Incorrect: You have to gain 15-kg weight.

Rule 16

The usage of non-standard units and symbols should always be avoided.

Correct: Concentration of NaOH in solution is 2 kmol m⁻³.

Incorrect: Concentration of NaOH in solution is 2 M, where M is molarity.

Rule 17

Numbers more than three digits are separated into groups of the three to reduce confusion. Spaces are used as a separator; commas and periods are avoided as a separator.

Correct: 34 923.23 N m⁻²

Incorrect: 34923.23 N m⁻²

Rule 18

Certain terms which have a different meaning in different languages or at different locations should be avoided. For example, like million, billion, trillion have different meanings.

Correct 10 µg g⁻¹

Incorrect: 10 ppm

Associated Articles

• SI units list
• SI units
• Significant figures