Chemistry · Concentration
PPM to molarity calculator.
Convert between ppm, ppb, mg/L and mol/L with the molar mass. Add a density if the solution is not dilute water.
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How to convert ppm to molarity
ppm is a mass ratio: milligrams of solute per kilogram of solution. Molarity is moles per litre of solution. In dilute water, 1 kg is about 1 L, so 1 ppm = 1 mg/L and 1 ppb = 1 ug/L. Then molarity = (mg/L / 1000) / molar mass.
Worked example 1. A water report lists 50 mg/L of nitrate as the NO3 ion (molar mass 62.004 g/mol). 0.050 g/L / 62.004 g/mol = 0.000806 mol/L, which is 0.806 mM. Check what the report means: some labs report nitrate "as N", which has a different molar mass (14.007 g/mol), and the same number then means a different molarity.
Worked example 2. 1 mM NaCl (58.44 g/mol) is 58.44 mg/L, or about 58.44 ppm in dilute water.
For a dense or non-water solution, ppm (mass) and mg/L (mass per volume) are no longer equal: mg/L = ppm x density in g/mL. Enter the density and the tool does that step. IUPAC advises writing units such as mg/kg or mg/L out in full instead of "ppm", because ppm does not say whether the ratio is by mass, mole or volume.
Frequently asked questions
How do I convert ppm to molarity?
Treat 1 ppm as 1 mg/L in dilute water, divide by 1000 to get g/L, then divide by the molar mass in g/mol. Molarity = ppm / 1000 / molar mass.
Is 1 ppm always equal to 1 mg/L?
No. It holds when the solution density is about 1.00 g/mL, as in dilute water. For other solutions, mg/L = ppm x density in g/mL.
How do I convert mol/L to ppm?
Multiply molarity by molar mass to get g/L, then by 1000 for mg/L. In dilute water that number is also the ppm.
What is the difference between ppm and ppb?
ppm is parts per million (mg per kg), ppb is parts per billion (ug per kg). 1 ppm equals 1000 ppb.
Sources
Checked
- OpenStax Chemistry 2e, 3.4 Other Units for Solution Concentrations (mass percent, ppm, ppb, 37.2% HCl example)
- Wikipedia: Parts-per notation (1 ppm = 1 mg/L in water at 1.00 g/mL; IUPAC advice to write units out)
- CIAAW: Abridged Standard Atomic Weights 2024 (molar masses)
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.