Relative quantities

Relative units

The most common relative unit is per cent:

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:

A common type of relative unit for concentration gives the unit value per million, billion or trillion:

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:

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' in Numbers). For ppt, trillion is one million million (10¹²).

When using the relative units weight for weight and weight for volume, use the form:

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).

Percentages

Express a percentage in one of the following ways:

Note: Per cent (2 words) is a term meaning units per 100 (e.g. 10 per cent or 10%); percentage is a reference to a quantity or rate expressed as the units per 100:

For a technical or scientific document that contains many percentages, use the symbol:

In a document with few numbers or for a general readership, per cent may be used:

Use % in tables and figures.

When presenting proportions or percentages, include the numbers involved, if practical – for example, compare the impressions given by these statements: