PPM Calculator: ppm, mg/L, % and Molarity
For water-based solutions, 1 ppm ≈ 1 mg/L because 1 L of dilute solution weighs about 1 kg. For dense or concentrated solutions, ppm (by mass) and mg/L differ by the solution's density.
What ppm means
Parts per million is a ratio: 1 part of solute per 1,000,000 parts of the whole. By mass, that's 1 mg per kg. Because a liter of dilute water solution weighs about 1 kg, 1 ppm ≈ 1 mg/L ≈ 1 µg/mL. The rest follow from powers of ten: 1% = 10,000 ppm, 1 ppm = 1,000 ppb, 1 g/L = 1,000 ppm.
To get molarity, divide mg/L by the molecular weight and by 1,000: M = (mg/L) ÷ (MW × 1000).
Worked examples
250 ppm sodium chloride: 250 mg/L = 0.25 g/L = 0.025%. With MW 58.44: 250 ÷ 58,440 = 0.00428 M (4.28 mM).
Making a 100 ppm standard: 100 mg of solute made up to 1 L. If your balance only reads to 0.01 g, weigh 1.000 g into 10 L, or better, weigh 100.0 mg on an analytical balance reading to 0.1 mg, or make a concentrated stock and dilute it with the dilution calculator.
Common mistakes
- Assuming ppm = mg/L for everything. It only holds for dilute water solutions. Seawater, syrups and solvents need a density correction.
- Mixing up ppm by mass and by volume. Gas concentrations (ppmv) are by volume and don't convert to mg/L the same way.
- Weighing tiny amounts on a coarse scale. Use the balance readability calculator to check your scale is fine enough.
FAQ
Is mg/kg the same as ppm?
Yes, by mass. It's the usual unit for solids such as soil, food and fertilizer.
How do I convert ppm to percent?
Divide by 10,000. 5,000 ppm = 0.5%. For full recipes in percent, see the percent solution calculator.
More tools: free lab calculators.
