Converting concentration units in water analysis (mg/l, µg/l, mol/l)
A mistake in units is one of the most common and most costly errors in working with water data - because different sources give concentrations in different ways. A few rules let you convert freely and avoid slips.
Basic mass conversions
- 1 mg/l = 1000 µg/l (a milligram = a thousand micrograms),
- ppm ≈ mg/l for water (1 mg per litre ≈ 1 part per million), and ppb ≈ µg/l,
- 1 g/m³ = 1 mg/l.
This is enough in most everyday situations.
Mass versus moles
Moving between mg/l and mol/l requires the molar mass of the substance:
concentration [mol/l] = concentration [g/l] ÷ molar mass [g/mol]
Example: for nitrates (NO₃⁻, ~62 g/mol) 1 mg/l ≈ 0.016 mmol/l. The same applies to phosphates, ammonia and so on.
Watch out for the “form” - N and P
The most common pitfall with nutrients: whether the result is given as the whole compound or as the element:
- nitrate nitrogen is written as N-NO₃ (the mass of the nitrogen alone) or as NO₃ (the whole ion),
- similarly N-NH₄ vs NH₄, P-PO₄ vs PO₄.
These values differ (e.g. NO₃ is about 4.4 times more than N-NO₃, because the mass of oxygen is added). Always check in which form the result is given - otherwise you are comparing two different things.
Practical rules
- Keep one unit and form throughout the series and record them explicitly.
- When importing from different sources standardise first, then compare.
- Remember the decimal separator when exporting (see export to Excel).
🧮 Tool: Concentration unit converter - mg/l, µg/l, mmol/l, ppm and the forms of nitrogen and phosphorus (NO₃ ↔ NO₃-N, PO₄ ↔ PO₄-P).
In practice
The fewest mistakes arise when each indicator has a unit assigned in advance. In LimnoLog you keep an indicator in the chosen unit consistently throughout the series, so comparisons and charts are unambiguous. On how to read results in general, we write in the article How to interpret water test results.
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