The most common relative unit is per cent (see Percentages):
Other relative units include ratios – for example, 4 people per thousand population (expressed as 4 per 1,000). Even if the denominator is 100, this type of ratio is not usually expressed as a percentage:
There were 6 accidents per 100 vehicles [rather than 6% of vehicles were involved in accidents]
A common type of relative unit for concentration gives the unit value per million, billion or trillion:
parts per million (ppm) parts per billion (ppb) parts per trillion (ppt)
If possible, avoid these types of units. If relative units are absolutely necessary, put the conversion in brackets after the absolute units at the first (or most defining) mention:
The limit of detection of the analysis was 2 µg/kg (2 ppb).
where:
- 1 ppm = 1 mg/kg, 1 mg/L, 1 µg/mL
- 1 ppb = 1 µg/kg, 1 ng/mL
- 1 ppt = 1 ng/kg, 1 ng/L, 1 pg/mL
Note: For ppb, billion refers to 1,000 million (109, not 1012; see Large numbers). For ppt, trillion is one million million (1012).
When using the relative units weight for weight and weight for volume, use the form:
wt/wt, wt/vol not w/w, w/v
All relative units should be given in full at the first use and spelt out in a list of shortened forms (if the document has one).
