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Parts per million (ppm)

Quantity: concentration ratio · Category: Concentration

Parts per million expresses a concentration ratio as one part of solute per one million parts of the mixture. It may be mass-based (mg of solute per kg of solution) or volume-based; for dilute aqueous solutions the two are numerically nearly identical because 1 L of water has a mass of about 1 kg. Always check which basis a source uses, since mass- and volume-based ppm differ in dense liquids and in gases.

Relation to SI

1 ppm = 1 mg/kg; ≈ 1 mg/L for dilute aqueous solutions

Examples

  • Drinking-water fluoride at 0.7 ppm ≈ 0.7 mg/L
  • Air pollutants such as CO are reported in ppm by volume
  • 1 ppm = 0.0001% by mass