Tool · free
Concentration unit converter - with nitrogen and phosphorus forms
Convert concentrations between mg/l, µg/l, mmol/l, µmol/l and ppm - and for nitrogen and phosphorus compounds also between the ionic and elemental form (e.g. NO₃ ↔ NO₃-N, PO₄ ↔ PO₄-P). That second conversion is the most common source of mistakes when reporting results.
Enter a value to see the conversion.
How it works
Converting between mass and molar units relies on the compound’s molar mass (mmol/l = mg/l ÷ M). Converting between the ionic and elemental form means multiplying by a mass ratio: the share of nitrogen or phosphorus in the molecule. For the most common forms:
- NH₄ → NH₄-N: × 0.776 (14.01 / 18.04)
- NO₃ → NO₃-N: × 0.226 (14.01 / 62.00)
- NO₂ → NO₂-N: × 0.304 (14.01 / 46.01)
- PO₄ → PO₄-P: × 0.326 (30.97 / 94.97)
We treat ppm as mg/l (water density ≈ 1 kg/l). Values are indicative - for precise laboratory work use the exact molar masses for your form.
Frequently asked questions
How does NO₃ differ from NO₃-N (nitrate nitrogen)?
NO₃ is the concentration of the whole nitrate ion, while NO₃-N is the same sample expressed as just the nitrogen in that ion. Conversion: 1 mg NO₃/l ≈ 0.226 mg NO₃-N/l (molar mass ratio 14.01 / 62.00). Mixing up these two values is the most common error when reporting nitrogen.
Is ppm the same as mg/l?
In dilute aqueous solutions water density is taken as ≈ 1 kg/l, so 1 ppm (mg/kg) ≈ 1 mg/l. For natural waters this approximation is practically exact; in very saline solutions density rises and the equality is no longer strict.
How do I convert mg/l to mol/l?
Divide the mass concentration by the compound’s molar mass: mmol/l = (mg/l) / M. E.g. 62 mg/l NO₃ ÷ 62.00 g/mol = 1 mmol/l. The calculator does this automatically for the selected compound.
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