How this calculator works
Parts per million is a ratio of one part in a million. Milligrams per litre is a mass of solute in a volume of solution. To go from a mass ratio to a mass concentration you need the density of the solution: 1 ppm of a solution that weighs 1 kg/L is 1 mg in 1 L, so it is 1 mg/L. For water and dilute aqueous solutions the density is close enough to 1 kg/L that the two are often treated as equal, but the equality is an approximation. Molarity adds one more step: divide the mass concentration by the molar mass and convert grams to moles.
Formula
mg/L = ppm x density (kg/L). ppm = (mg/L) / density. Molarity (mol/L) = (mg/L) / (1,000 x molar mass in g/mol). The 1,000 converts milligrams to grams.
Worked example
1 ppm in water at 1 kg/L is 1 mg/L. 10 ppm at 1.02 kg/L is 10.2 mg/L. 40 mg/L of calcium at 40.078 g/mol is 0.00099805 mol/L, or 0.99805 mmol/L.
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Frequently asked questions
When can I treat ppm as mg/L?
Only when the solution density is about 1 kg/L, which is true for dilute aqueous solutions such as drinking water. A concentrated brine or an organic solvent needs its own density, and the calculator asks for it.
Which molar mass should I use?
Use the molar mass of the dissolved substance, not the solution. The default of 18.01528 g/mol is water and is useful only when the solute itself is water or when you are checking the arithmetic.
Does ppm refer to mass or moles here?
This page treats ppm as a mass fraction, which is the usual convention in water analysis. Some branches of chemistry use ppm as a mole fraction or a volume fraction; check the source before applying the result.