Hardness of water - these values ​​you should know


In Germany, the specification in German degrees of hardness (° dH) is often decisive. However, the chemically and legally correct unit of hardness is mmol / l. In addition, often other units are common. Which relation the individual degrees of hardness have to each other and how the individual hardness stages of water are divided is therefore explained in detail in this article.

Units of measure for water hardness

In practical households, the permanent hardness of the water is always given. This is important because the temporary water hardness provides significantly different values.

German degrees of hardness

The units of measurement still in use in Germany are the so-called German degrees of hardness (° dH). This unit is based on the following calculation:

1° dH corresponds to exactly 10 mg of dissolved CaO in one liter of tap water. Only the calcium content is given, the other alkaline earth metals are not included in the calculation.

The content of MgO can be specified separately in addition to the German degrees of hardness. In many cases, this is waived.

Correct specification

The correct indication, as required by law, takes into account all available alkaline earth metals in the water. The content of magnesium must therefore also be taken into account. It is thus the so-called total hardness of the water specified in the water hardness.

According to the SI units valid in the field of technology and natural sciences, the specification must be made in mmol / l.

French degrees of hardness

The French degrees of hardness (° fH) are the most commonly used measure not only in France but also in Switzerland. They are calculated in a different way.

Other units

In the English-speaking countries, either English grades (° e) or ppm CaCO3 are common units. For the German-speaking countries they have no meaning.

Conversion of units into each other

German and French degrees of hardness can not be converted exactly in mmol / l. The possibly fluctuating content of other alkaline earth metals, which contribute to the overall hardness in the legal sense, makes an exact calculation impossible.

Roughly approximate, however, it can be assumed that 1° dH corresponds to approximately 0.1783 mmol / l in most waters, 1° fH corresponds to approximately 0.1 mmol / l.

Categories of water hardness grades

  • soft water (up to 1.3 mmol / l or 7.3° dH)
  • medium-hard water (1.3 to 2.5 mmol / l or 7.3° dH to 14° dH)
  • hard water (2.5 to 3.8 mmol / l or 14° dH to 21.3° dH)
  • very hard water (above 3.8 mmol / l or above 21.3° dH)

Tips & Tricks

For an exact specification of the water hardness, therefore, a test procedure must be used, which provides the right unit from the outset.


Video Board: Testing Water for Hardness - Boiling Point