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). It is a term with several variants in meaning, so the meaning should be made clear if this term is used.

According to the U.S. National Institute of Standards and Technology (NIST) Guide for the Use of the International System of Units (SI), "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." NIST's Guide for the Use of the International System of Units (SI) (http://physics.nist.gov/Pubs/SP811/sec07.html#7.10.3) has examples of alternative expressions.

Senses of parts 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) by mass is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg, which unlike ppm is unambiguous.

• By mass:
  • one milligram in a kilogram is 1 ppm by mass.
  • one gram in a metric tonne.
• By volume:
  • one millilitre (or cubic centimetre) in a cubic metre (or kilolitre) is 1 ppm by volume. For most gases (those behaving much like an ideal gas) this is numerically equivalent to µmol/mol on the basis of molecules (not atoms). See Avogadro's law.
• By mass/volume ratio for dilute aqueous solutions (ppm w/v or ppm m/v):
  • 1 litre (L) of water has mass of approximately 1 kg¹, so 1 mg/L is, loosely speaking, 1 ppm, for small concentrations in a water solution².
• By number of particles or moles:
  • one micromole per mole can also be called 1 ppm.

Use

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

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

Inexact analogues

• one square centimeter in 1000 square feet is about .95 ppm
• one two-parent, two-child family in a city of about 4 million people is roughly 1 ppm
• one CD in the 1.57-million disc³ FreeDB catalogue is nearly 2/3 ppm

Related units

• Percent (%) 1 part in 100. 1% is 10,000 ppm.
• Per mille (‰) 1 part in 1,000. 1‰ is 1000 ppm.
• Parts per billion (ppb).
• Parts per trillion (ppt).

Notes

1. Exactly one kg of pure water at maximum density (~4°C) and standard pressure was the definition of a litre from 1901 to 1964; today the litre is defined as exactly 1 dm³, the distinction being whether it is calibrated to the international standard kilogram (http://www.sizes.com/units/kilogram.htm) or the international standard meter (http://www.sizes.com/units/meter.htm), which are defined without reference to one another.
2. Properly speaking it is approximately 1 ppm by mass or by weight in solution. When solids dissolve, they can increase or decrease the total volume they occupy, and even increase or decrease the total volume of the solution. Adding 1 ppm by weight will rarely produce a solution that is 1 ppm by volume to the same precision. The notation ppm w/v or ppm m/v demonstrates the exact nature of the ratio and is therefore the most precise.
3. As of January 2005.