Gagné 2021, pp. 22-23

Ions are measured as mass per volume, mg/L, often written ppm on American bottles, which Gagné considers misleading since it is not a number of ions per million molecules. Chemists prefer to count ions: moles per liter, or “ppm as CaCO₃”, the concentration by mass one would get by replacing the ion with a molecule of calcium carbonate, a strange but convenient way of obtaining a quantity that does not depend on the mass of the species.

This is the unit adopted by the SCA. One ppm as CaCO₃ equals 0.056 German degrees, 0.01 mmol/L of calcium plus magnesium or 0.02 mmol/L of bicarbonate, 0.40 mg/L of calcium, 0.24 mg/L of magnesium, 1.22 mg/L of bicarbonate. One French degree equals 10 ppm CaCO₃; one German degree, 17.85.

Gagné 2021, pp. 22-23, conversion table; Wellinger, Smrke and Yeretzian, SCA Water Quality Handbook (2018)