Calculate water hardness - this works

For the water hardness, there are different dimensions, different units and different methods for calculation. Everything you need to know about the calculation and conversion of the water hardness will therefore learn in this post.

Temporary and permanent hardness

For the calculation, it is essential whether one wants to specify the so-called temporary hardness or the permanent hardness of the water.

  • Also read - permanent water hardness
  • Also read - temporary water hardness
  • Also read - Magnetic water softening - does that work?

There are different measuring methods for both values, which therefore produce different units in the result. For the usual use in relation to its own drinking water from the line or from other sources, however, the permanent hardness or non-carbonate hardness is almost always crucial.


The traditional units for the indication of the water hardness are different in each country. In Germany and Austria, the unit of use of German hardness levels (° DH) is often used, while in France and Switzerland, the French hardness levels (° FH) are decisive.

Anglo-Saxon units

In addition, even English hardness levels (° E) and the USA also the unit PPM CaCo3 spread extended dimensions. A direct comparison of the units of units into each other is not exactly possible because they are based on different measurement methods.

German hardness and chemically correct information

1 ° DH means the content of 10 mg CaO per liter of water, all other alkaline earth metals are not considered in the unit ° dhwise, but specified separately.

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

This also corresponds to the SI units alone in the technical and scientific field. A conversion of ° DH in mmol / L is not exactly possible because of the possibly fluctuating content of magnesium and other alkaline earth metals, but only approximately possible.

As a rough conversion factor one can take a conversion ratio of 1: 0.1783 for German hardness levels for most watering. (1 ° DH equals around 0.1783 mmol / l).

In French hardness, the ratio is about 1: 0.1.

Tips & Tricks If you use measuring instruments or other measurement methods, you should make sure that the specification is done in mmol / l - a conversion from ° DH or ° FH is not exact possible.