Units & symbols

The International System of Units (SI) is used. Six base units are given in table 1; two supplementary units are given in table 2; and twenty-five derived units are given in table 3.

Symbols for units should be in roman type in all contexts and separated from each other and from the numeral by a fixed thin space (exception: no space between numeral and ‘°C’).

• SI prefixes, given in table 4, precede the SI unit: ‘mg’, ‘ns’, ‘µA’, ‘MHz’, ‘kPa’. Note there is no space separating the prefix from the unit
• Note the preferred spelling of certain units: `metre', `millimetre', `gram', `kilogram', etc.
• Units that are in common everyday usage (e.g. years, days) should be given in full. Use ‘d’ for ‘day(s)’ and ‘yr’ for ‘year(s)’ in compound units (e.g. 5 d yr⁻¹). Abbreviate ‘year’ to ‘yr’ when referring to a unit of time e.g. 17.3 yr; 8 yr⁻¹. Use ‘year’ when referring to a specific period of time, e.g. the year 2007, it lasted for 3 years, etc.
• Ma is used when referring to specific point in geological time or to say how long ago an age began (e.g. the K-T event at 65 Ma). Use Myr when referring to a time span or how long the age lasted for.
• Second, minute, hour, day, year should be given in numerals.
• Weeks and months (if under 10) should be in words (except in a range or list numbers that goes above 10, or it is used as a measure (e.g. 1 week °C⁻¹)).
• Only use the abbreviated form (yr, d, s, min, h) of year, day, second, minute, hour in body text when a valued experimental (as opposed to factual) measure is being referred to. For instance, ‘It took 2000 years for the lake to form’ (i.e. this is factual) versus ‘Growth occurred at a rate of 2 mm yr⁻¹’ or ‘t = 1000 yr’ (experimental).
• Repetition of units: remove first instance, e.g. ‘4 × 6 mm’.

Table 1. Names and Symbols for the SI base units

Table 2. Names and Symbols for the SI supplementary units

Table 3. Names and Symbols fo SI-derived units

Table 4. SI prefixes

See ‘Quantities, Units, and Symbols’ (The Royal Society, 1975) for further details.