Calculate water hardness - this is how it works

For the water hardness there are different dimensions, different units and different calculation methods. Everything that you need to know about the calculation and conversion of the water hardness is therefore explained in this article.

Temporary and permanent hardship

First of all, it is decisive for the calculation whether one wants to specify the so-called temporary hardness or the permanent hardness of the water.

For both values, there are different measuring methods, which therefore also produce different units in the result. However, for practical use in relation to one's own drinking water from the line or from other sources, the permanent hardness or non-carbonate hardness is almost always decisive.


The traditional water hardness units are different in each country. In Germany and Austria, the unit of measure German degrees of hardness (° dH) is often used, while in France and also in Switzerland the French degrees of hardness (° fH) are decisive.

Anglo-Saxon units

In addition, there are also English degrees of hardness (° e) and in the United States also the unit ppm CaCO3 common dimensions. A direct comparison of the units of measurement is not exactly possible because they are based on different measurement methods.

German degrees and chemically correct indication

1° dH means the content of 10 mg CaO per liter of water, all other alkaline earth metals are not taken into account in the unit° dH, but specified separately.

In legal regulations, however, today a total specification of all alkaline earth metals in water, the so-called total hardness, in a chemically correct indication in mmol / l (milli moles per liter) is stipulated as binding in the determination of hardness.

This also corresponds to the SI units valid in the technical and scientific fields alone. A conversion of° dH in mmol / l is not exact because of the possibly fluctuating content of magnesium and other alkaline earth metals, but only approximately possible.

As a rough conversion factor, one can assume a conversion ratio of 1: 0.1783 for German degrees of hardness for most waters. (1° dH corresponds to about 0.1783 mmol / l).

For French degrees of hardness, the ratio is about 1: 0.1.

Tips & Tricks

When using measuring instruments or other measuring methods, it is essential to make sure that the information is given in mmol / l - a conversion from° dH or° fH is not exactly possible.