PPM to Molarity Calculator
Convert parts per million to molarity and back — the conversion behind every water-quality and trace-analysis result.
For a dilute aqueous solution, 1 ppm is 1 mg/L.
How the ppm to molarity calculator works
For a dilute aqueous solution, 1 ppm is 1 mg/L. Dividing by the molar mass in g/mol gives molarity: M = ppm ÷ (1000 × molar mass).
The assumption is that the solution's density is 1 kg/L. That holds well for dilute water samples and fails for brines, organic solvents and anything concentrated — which is exactly where people apply it anyway.
Formula: M = ppm / (1000 × molar mass)
Worked examples
| Inputs | Result | Note |
|---|---|---|
| 250 ppm as CaCO₃ (hard water) | 0.00249775 | 2.5 mM |
| 1 mM back to ppm | 100.09 | 100.09 ppm |
| Lead at 10 ppb | 4.8263e-8 | 48 nM |
FAQFrequently asked questions
Is 1 ppm always 1 mg/L?
Only when the solution has a density of 1 kg/L, which is a good approximation for dilute water. It fails for brines and organic solvents.
What is 250 ppm hardness?
Expressed as calcium carbonate, that is 2.5 mM — firmly in the "hard water" range.
How do I go from molarity to ppm?
Multiply by the molar mass and by 1,000.
What about ppb?
A thousandth of a ppm — 1 µg/L in water. It is the usual scale for heavy metals and pesticide residues.
Why is hardness quoted "as CaCO₃"?
Because hardness comes from several ions. Expressing them all as an equivalent amount of calcium carbonate gives one comparable number.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.