Hardness is defined as the total dissolved calcium and magnesium content of the water - and is dependent on the type of water in the subsoil where each waterworks draws water.

In Denmark, hardness is measured and expressed in German degrees of hardness (°dH = grad Deutsche Härte). One degree of hardness is equal to 10 mg of dissolved calcium oxide per liter or 7.19 mg of dissolved magnesium oxide per liter. 

Hardness is categorized according to this scale:

• 0-8 °dH = soft and very soft water
• 8-12 °dH = Medium hard water
• 12-18 °dH = Fairly hard water
• 18-24 °dH = Hard water
• 24-30 °dH = Very hard water
• 30 °dH or more = Extremely hard water

Where is the water hardest?

Dragør is the municipality in Denmark with the highest degree of hardness - i.e. where there is the most calcium in the water. Here, the °dH value is above 30, which is labelled as 'extremely hard'. 

In general, the water is hardest on Zealand, Falster, Lolland, Møn and Funen. Jutland has the most soft and medium-hard water, but there are also areas where the water is hard in the western part of Denmark.