For dilute aqueous solutions, parts per million and milligrams per litre are effectively equal because one litre of water weighs about one kilogram. So 1 ppm ≈ 1 mg/L.
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Frequently asked questions
Is ppm the same as mg/L?
For water at normal temperatures, yes — 1 ppm ≈ 1 mg/L. For other solvents or gases the density differs.
When does it not equal?
For dense or light solutions, or gases, you must account for the actual density.