Drinking water hardness (°dH, mmol/l)

Water hardness expresses how much dissolved calcium and magnesium the water contains. Czech drinking water Decree No. 252/2004 Coll. gives a recommended value of 2-3.5 mmol/l for their sum. The decree has no separate 'hardness' indicator, but water operators often also give German degrees (°dH); 1 mmol/l is about 5.6 °dH.

How to read the value

• The recommended range of 2-3.5 mmol/l is roughly 11-20 °dH and is set mainly on health grounds, not for the sake of appliances.
• Terms such as 'soft', 'medium hard' or 'hard' are not officially defined; each operator or manufacturer uses its own scale.
• The figure applies to water from the public supply; a private well needs its own analysis.

What it means for a household

Harder water forms more limescale in kettles, washing machines, water heaters and hot water exchangers, for example in a heat pump. With very hard water a softener is often installed. Very soft water, on the other hand, can be more corrosive to metal pipes and, according to the National Institute of Public Health, is not ideal for health either.

Where to find the data

Under Section 3a of the Public Health Protection Act, water operators must publish current information on the quality of the water they supply, usually on their website. Where a municipality has several sources, values may differ by area, so it is worth finding out which supply or well an address belongs to and how old the data are. The network operator overview or the water and sewage bill shows who runs the supply.

Frequently asked questions

Is hard water bad for health?

For calcium and magnesium the decree sets only recommended values, not limits. According to the National Institute of Public Health, the literature links health problems only to very hard water above 5 mmol/l, and even there the cause is unclear. For ordinary tap water, hardness is mainly a technical and taste indicator.

How do I convert degrees to mmol/l?

Divide the °dH value by 5.6. Example: water with 16.8 °dH has about 3.0 mmol/l, which is within the recommended range.