PPM to Molarity Converter
Convert parts per million to molarity. Enter the concentration in ppm, the solute's molar mass in g/mol and the solution density in kg/L (use 1.0 for dilute aqueous solutions) to get molarity in mol/L, plus mM and mg/L.
Example: with Concentration (ppm) 100 · Molar mass (g/mol) 58.44 · Solution density (kg/L) 1 → Molarity: 0.001711 M (mol/L).
- In millimolar (mM)1.7112 mM
- Mass concentration100 mg/L
Computed by the calculator below using its default values. Change any input to see your own numbers.
For a dilute aqueous solution ppm is essentially mg/L. Molarity M = ppm × density ÷ (1000 × molar mass), because ppm (mg/L) ÷ 1000 gives g/L, then ÷ molar mass gives mol/L.
From ppm to moles per liter
Parts per million by mass means milligrams of solute per kilogram of solution. For water-based solutions near room temperature the density is about 1 kg/L, so 1 ppm is very nearly 1 mg/L. Converting to molarity is then two divisions: turn mg/L into g/L by dividing by 1000, then into mol/L by dividing by the molar mass. Put together, M = ppm × density ÷ (1000 × molar mass).
The molar mass is what makes ppm and molarity disagree. A hundred ppm of a light ion is more moles than a hundred ppm of a heavy one, because the same mass holds more of the lighter particles. For non-dilute or non-aqueous solutions, set the density to the real value so the mg/L step stays accurate.
How it’s calculated
M = (ppm × density) / (1000 × molar mass). ppm is taken as mg per kg of solution; multiplying by density in kg/L gives mg/L, dividing by 1000 gives g/L, and dividing by molar mass (g/mol) gives mol/L. Density defaults to 1 kg/L for dilute aqueous solutions.
ppm is a mass-based concentration (mg/kg). For dilute water solutions density ≈ 1 kg/L makes ppm and mg/L interchangeable; adjust density for concentrated or non-aqueous solutions.
1 ppm converted to molarity (density 1 kg/L)
| Species | Molar mass | Molarity of 1 ppm |
|---|---|---|
| Sodium ion, Na+ | 23.0 g/mol | 4.35 × 10⁻⁵ M |
| Calcium ion, Ca²⁺ | 40.1 g/mol | 2.50 × 10⁻⁵ M |
| Chloride ion, Cl⁻ | 35.45 g/mol | 2.82 × 10⁻⁵ M |
| Sodium chloride, NaCl | 58.44 g/mol | 1.71 × 10⁻⁵ M |
Computed as M = ppm / (1000 × molar mass) at density 1 kg/L; rounded.
Common mistakes
- Forgetting the factor of 1000 that converts mg/L to g/L before dividing by molar mass.
- Using the wrong density for concentrated or non-water solutions, which throws off the mg/L step.
- Treating ppm as a mole ratio; ppm here is a mass ratio, so molar mass matters.
- Confusing ppm by mass with ppm by volume for gases — this converter is for solutions.
Frequently asked questions
How do you convert ppm to molarity?
Use M = ppm × density ÷ (1000 × molar mass). For dilute water solutions density is about 1 kg/L, so it reduces to ppm ÷ (1000 × molar mass).
Is 1 ppm the same as 1 mg/L?
For dilute aqueous solutions, yes, because water's density is close to 1 kg/L. In general 1 ppm equals 1 mg per kg of solution, which becomes mg/L after multiplying by density.
Why do I need the molar mass?
Molarity counts moles, and moles depend on mass divided by molar mass. Two solutions at the same ppm have different molarities if their solutes have different molar masses.
What density should I use?
Use 1 kg/L for dilute water-based solutions. For concentrated brines, acids, or non-aqueous solvents, enter the actual solution density so the mg/L conversion is correct.