Parts per million

Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. The ppm value is equivalent to the absolute fractional amount multiplied by one million (10⁶).

Table of contents

1 Illustrations of one part per million 2 Use 3 Another parts per units

Illustrations of one part per million

The metric system is the most convenient way to express this since metric units go by steps of ten, hundred and thousand. For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own abbreviation -- mg/kg. Using our abbreviations, one ppm equals one mg/kg.

• one milligram in a kg is 1 ppm (by mass).
• one gram in a metric tonne.
• 1 liter (l) of pure water at 4°C and one standard atmosphere pressure weighs exactly 1 kg, so 1 mg/l is 1 ppm.
• one millilitre in a cubic metre (or kilolitre).
• In a percentage, 0.1% are 1000 ppm or 1% are 10,000 ppm.

Use

Examples of situations where parts per million are an appropriate measure include:

• relative abundances of rare elements in the earth's crust
• concentrations of pollutants in the environment

Another parts per units

Similar, even smaller measures are:

• Parts per billion (ppb).
• Parts per trillion (ppt).