Water hardness - units and conversions
Water hardness is one of those parameters where the units cause the most confusion - the same result is given in several ways. It is worth knowing what hardness means and how to convert freely between units.
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
The most important thing is to stick to one unit in your monitoring and record it explicitly - otherwise mistakes in comparisons are easy (see exporting data to Excel). In LimnoLog you keep hardness as an indicator with a chosen unit, consistently throughout the series.
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