Water hardness - units and conversions
With water hardness, the units cause the most confusion: the same laboratory result arrives once in German degrees, once in mg CaCO₃/l, once in mmol/l. This page shows what hardness means and how to convert between units without guessing.
What hardness is
Hardness is the content of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. It results mainly from the geology of the catchment (water from limestone areas is harder) - a good example of geochemical background. A distinction is made between total hardness, carbonate (temporary) and non-carbonate (permanent).
Units and conversions
All hardness units describe the same thing on a different scale, so they convert exactly, with no assumptions about the type of water:
| 1 unit | mg CaCO₃/l | °dH | mmol/l | mval/l (meq/l) |
|---|---|---|---|---|
| 1 mg CaCO₃/l (= 1 ppm) | 1 | 0.056 | 0.00999 | 0.0200 |
| 1 °dH | 17.848 | 1 | 0.178 | 0.357 |
| 1 °fH | 10 | 0.560 | 0.0999 | 0.200 |
| 1 mmol/l | 100.09 | 5.61 | 1 | 2 |
| 1 mval/l (meq/l) | 50.04 | 2.80 | 0.5 | 1 |
The German degree is the most common in Central Europe, the French one is handier in arithmetic (exactly 10 mg CaCO₃/l). mmol/l describes the sum of Ca²⁺ and Mg²⁺ ions, mval/l the same sum in equivalents - hence the fixed relation 1 mmol/l = 2 mval/l (both ions are divalent).
When you only have calcium and magnesium
Total hardness follows directly from the concentrations:
hardness [mg CaCO₃/l] = 2.497 × Ca [mg/l] + 4.118 × Mg [mg/l]
The coefficients are the ratios of the molar mass of CaCO₃ to Ca and to Mg (Standard Methods,
method 2340 B). The same formula is used by the water hardness calculator and by the
application itself - the calculated TH_calc parameter in LimnoLog rests on these two numbers,
so the article, the calculator and your project return one value.
How to read the result
| Hardness | °dH | mg CaCO₃/l | mmol/l |
|---|---|---|---|
| soft | < 8 | < 143 | < 1.4 |
| moderately hard | 8-14 | 143-250 | 1.4-2.5 |
| hard | 14-21 | 250-375 | 2.5-3.7 |
| very hard | > 21 | > 375 | > 3.7 |
This is a descriptive scale, not a quality standard. Hardness is not an indicator of pollution - high hardness in limestone catchments is natural and does not mean worse water. In drinking-water regulations hardness usually appears as an indicator parameter (a recommended range, not a limit). The same four classes are used by our water hardness calculator.
Sources
- The formula for hardness from Ca and Mg and the conversions to CaCO₃: Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF), method 2340 B.
- Titrimetric determination (EDTA): ISO 6059 - the sum of calcium and magnesium.
- Molar masses used in the conversions: CIAAW.
In practice
In your monitoring, stick to one unit and record it with every result - otherwise comparisons between series end in error (see exporting data to Excel). In LimnoLog an indicator has one chosen unit for the whole series.
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