Cochrane Style Manual
Units and systems of measurement

In this section: Standard units | Commonly used units | Prefixes for SI units | General guidance on SI units

Standard units

The International System of Units/Le Système International d’Unités (SI) is the standard metric system of measurement. This system is made up of SI base units (the foundation units, e.g. metre), derived units (e.g. square metre), and non-SI units that are accepted for use within the SI (e.g. minute).

Commonly used units

This table lists SI units and other units that are often used in Cochrane Reviews. The full list of SI units and further information is available from the International Bureau of Weights and Measures (BIPM) and the NIST Reference on Constants, Units, and Uncertainty.

Unit name

Symbol

Type

kilogram

kg

base unit

microgram

µg

base unit

metre

m

base unit

second (unit of time)

s

base unit

cubic metre

m3

derived unit

degree Celsius

°C

derived unit

metre per second

m/s

derived unit

square metre

m2

derived unit

day

d

non-SI unit

degree

°

non-SI unit

hour

h

non-SI unit

litre

Note: the BIPM adopted the symbol ‘l’ in 1879; it then adopted the alternative ‘L’ in 1979 in order to avoid the risk of confusion between the letter ‘l’ and the number ‘1’.

L

non-SI unit

minute (unit of time)

min

non-SI unit

minute (measurement of angle)

non-SI unit

second (measurement of angle)

non-SI unit 

Prefixes for SI units

This table includes the SI prefixes commonly used in Cochrane Reviews.

Factor

Name and symbol

Example

10-1

deci (d)

decilitre (where ‘litre’ is the base unit)

10-2

centi (c)

centimetre (where ‘metre’ is the base unit)

10-3

milli (m)

milligram (where 'gram' is the base unit)

10-6

micro (µ)

microlitre

10-9

nano (n)

nanogram

 

General guidance on SI units

SI units and their derivatives should follow the style conventions listed below. Unlike most abbreviations and acronyms, it is not necessary to define the full unit name on first use.

These are a selection of style conventions from NIST and BIPM (see links above). Cochrane Reviews may deviate from some of the style conventions due to the nature of Cochrane Review production; for example, Cochrane Reviews use commas to separate digits into groups of three (e.g. 150,739) instead of thin, fixed spaces (150 739).

Guidance

Correct

Incorrect

Unit symbols are unaltered when plural

10 mg

10 mgs

Unit symbols are not followed by a full stop, except when followed by normal sentence punctuation

I added 60 µg of salt.

I added 60 µg. of salt.

The unit symbol to which a numerical value belongs, and the mathematical operation that applies to the value of a quantity, should be clear.

20 °C to 30 °C

123 g ± 2 g

20 °C-30 °C
20 to 30 °C

123 ± 2 g
(123 ± 2) g

Values of quantities: use Arabic numerals plus symbols for units

m = 5 kg
 

the current was 15 A

m = five kilograms
m = five kg

the current was 15 amperes

Put one space between the numerical value and the unit symbol. Do not put a space between a prefix and the unit symbol.

Note: except in the case of superscript units for angles or degrees (e.g. 2° 3').

2 s

25 nL

2s

25 n L

When a value with unit is used as a modifier before a noun, write out the name of the metric quantity and use a hyphen between the numeral and unit.

a 2-second delay

a 20-liter container

a 2-s delay

a 20 L container

Do not mix information with unit symbols or names

the water content is 20 mL/kg

20 mL H2O/kg

20 mL of water/kg

Informal references to non-SI units, such as a historical quotes using inches, are acceptable depending on the context.

It took five hours to travel 10 miles in 1945.

It took five hours to travel 10 miles (16.09 km) in 1945.