mg/L (Milligrams per Litre)

Milligrams per litre (mg/L) is the standard unit used in water quality reporting. It expresses the mass of a substance dissolved in one litre of water.

Equivalences

• 1 mg/L = 1 ppm (parts per million)
• 1 ug/L = 1 ppb (parts per billion) = 0.001 mg/L
• 1 mg/L means 1 milligram of the substance is present in every litre of water

Context

To put these numbers in perspective:

• Fluoride at 1 mg/L = 1 milligram per litre = about 1 grain of salt in a litre
• Lead at 0.01 mg/L = 10 micrograms per litre = the ADWG limit
• PFOS at 0.00007 mg/L = 70 nanograms per litre = parts per trillion territory

Why it’s used

mg/L is used because it works across the wide range of concentrations found in drinking water — from bulk minerals like sodium (10–100+ mg/L) to trace contaminants like lead (0.001–0.01 mg/L).