Parts-per notation

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.

Illustrations of one part per million:

• one gram in a metric tonne (one ppm by mass)
• one millilitre in a cubic metre (one ppm by volume)

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

See also:

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