Parts per Million
A concentration unit expressing one part of solute per million parts of solution by mass or volume.
Formula / equation
ppm = (m_solute / m_solution) × 10⁶
Unit
ppm
In depth
For dilute aqueous solutions, where one litre weighs almost exactly one kilogram, 1 ppm conveniently equals 1 mg per litre. The unit is standard for trace contaminants, water hardness, atmospheric gases and permissible exposure limits.
Examples in the real world
Atmospheric CO₂ above 420 ppm; a drinking-water fluoride limit of 1.5 ppm.