milligram per liter
DensityConcentrationexact by definition
The milligram per litre is exactly 10⁻³ kilograms per cubic metre. It is the universal reporting unit of water analysis: hardness, alkalinity, chloride, iron and every regulated contaminant in drinking water are given in mg/L.
Watch out: In dilute aqueous solution mg/L and ppm are treated as interchangeable, and for water near 1000 kg/m³ that is accurate to well under a percent. The equivalence fails for anything else. In a brine, in a solvent, in soil or in air, ppm by mass and mg/L differ by the density, and in air quality work "ppm" usually means by VOLUME, which is a different quantity again.
| 1 mg/L (Density) | 0.001 kg/m³ |
| 1 mg/L (Concentration) | 0.0001 % |
The milligram per litre is treated here as 10⁻⁴ percent, equal to one part per million, which holds for any solution dilute enough that a litre still weighs a kilogram. It is the unit of nearly every water analysis: hardness, alkalinity, chloride, iron and total dissolved solids all arrive in mg/L.