Converting concentration units in water analysis (mg/l, µg/l, mol/l)

Knowledge base · August 1, 2026

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

The equality ppm = mg/l is an approximation based on a water density of ≈ 1 kg/l. For fresh water at typical temperatures the error is below 1% and has no practical meaning; for brines and hot water it does.

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 by a fixed factor equal to the ratio of molar masses:

An example of the scale of the error: a result of 10 mg/l NO₃ is 2.26 mg/l N-NO₃ - confusing the forms gives a more than fourfold difference, that is a different answer about the state of the water, not a rounding issue. The same molar masses are built into our concentration unit converter, so both sides agree.

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).

Where these molar masses come from

The atomic masses come from IUPAC tables (Atomic weights of the elements, updated periodically - hence the small differences in the third decimal between sources): CIAAW - Atomic weights of the elements. For surface and drinking water, rounding to three significant figures is entirely sufficient - the analytical uncertainty of the method is an order of magnitude larger than the difference between table versions.

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.