What is hard water or water hardness?

Water hardness indicates the concentration of dissolved foreign substances in water, particularly calcium and magnesium. In Germany, it is measured in degdH (degrees of German hardness), while North America uses grains per gallon or ppm. Tap water typically ranges from 6-15 grains per gallon (soft to moderate hardness), though well water in farming regions can exceed 25 grains per gallon or even reach over 100 grains.

Water hardness develops as rainwater passes through different soil and rock layers, absorbing water-soluble substances. In regions with limestone or gypsum-based ground, tap water becomes particularly hard and prone to limescale buildup in pipes and machines.

The minerals in hard water, magnesium and calcium, are important for human health and are not harmful when consumed. However, under certain conditions these substances can precipitate out, forming visible limescale in kettles and on fixtures, or invisibly settling in pipes. This can reduce water pressure, diminish heating efficiency, and block valves or pipe sections. Appliances using heated water, such as washing machines, dishwashers, and coffee makers, are particularly susceptible to calcification.

Water hardness information can typically be obtained from municipal utilities. Test strips are also available for determining hardness, particularly useful for private well owners.