**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 (10^{6}).

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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.

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

Similar, even smaller measures are: