PPM to Molarity Calculator
Convert a concentration in parts per million to molarity (mol/L) using the solute molar mass and the solution density.
How it works
Molarity (mol/L) = (ppm × density) ÷ (1000 × molar mass) · with density in g/mL and molar mass in g/mol
Parts per million by mass means milligrams of solute per kilogram of solution. To reach molarity you first convert that to milligrams per litre by multiplying by the solution density, then to grams per litre, then divide by the molar mass to get moles per litre. For dilute aqueous solutions the density is close to 1 g/mL, so ppm and mg/L are almost interchangeable — but the density term keeps the result correct for denser brines and solvents.
Worked example
100 ppm of NaCl (molar mass 58.44 g/mol) in water (density 1.0 g/mL): 100 × 1.0 = 100 mg/L = 0.100 g/L; 0.100 ÷ 58.44 = 0.001711 mol/L, or 1.71 mmol/L.
Sources: Parts-per notation — Wikipedia
Frequently asked questions
Is ppm the same as mg/L?
They are equal only when the solution density is exactly 1 g/mL, as with dilute water solutions. ppm is a mass ratio (mg per kg), while mg/L is per unit volume; the density converts between them.
Why do I need the molar mass?
ppm is a mass-based unit but molarity counts moles. Dividing the mass concentration by the molar mass converts grams of solute into moles, which is what molarity requires.
What density should I use?
For dilute aqueous solutions use 1.0 g/mL. For concentrated brines, acids or non-aqueous solvents use the measured solution density, since it can differ from water by 10% or more.
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This calculator is for educational purposes. Double-check important results before acting on them.