Hard water: harmless to health, technically relevant
Hard water is harmless to health but takes its toll on appliances. Hardness ranges, consequences and which filters really reduce limescale.
Hard water contains dissolved calcium carbonate (CaCO₃). As drinking water, it is harmless: calcium and magnesium are minerals, and the Trinkwasserverordnung (German Drinking Water Ordinance) sets no limit value for water hardness. The consequences show up in technical equipment. From around 14 °dH, limescale builds up in washing machines, coffee machines and pipes, and energy consumption rises. This guide shows the hardness ranges and the specific damage. It then covers the filter methods that measurably reduce limescale.
What defines hard water
Limescale in tap water is dissolved calcium carbonate (CaCO₃), a compound of calcium, carbon and oxygen. One millimole of it per litre corresponds to 5.6 degrees of German hardness. The substance gets into the water as rainwater seeps through the ground: carbonic acid dissolves calcareous rock, and calcium and magnesium go into solution as ions.
The everyday term refers to two states of the same substance. Dissolved in water, calcium carbonate is invisible and has little taste. It becomes solid when heated and when water evaporates: as a white stain on the tap, as scale in the kettle. The same reaction builds up stalactite caves over thousands of years.
This establishes the most important point. Calcium carbonate is one of the natural constituents of drinking water, not a pollutant. Reducing hardness protects appliances. It does not improve the quality of the drinking water.
Hardness ranges: soft, medium and hard
Three hardness ranges are defined by law. The Wasch- und Reinigungsmittelgesetz (WRMG, German Detergents and Cleaning Products Act) sets them out in § 9 paragraph 2, measured in millimoles of calcium carbonate per litre. In everyday use, the unit degrees of German hardness (°dH) is widespread. One millimole per litre corresponds to around 5.6 °dH.
| Hardness range | Legal definition | Equivalent to |
|---|---|---|
| soft | below 1.5 mmol/l calcium carbonate | below 8.4 °dH |
| medium | 1.5 to 2.5 mmol/l calcium carbonate | 8.4 to 14 °dH |
| hard | above 2.5 mmol/l calcium carbonate | above 14 °dH |
The degree of hardness depends on the underlying geology. Limestone and gypsum rock produce hard water, granite and sandstone rather soft or moderately hard water. As a rough trend: northern Germany is softer, southern Germany harder. However, there is no official ranking of the cities with the hardest water, because the values vary even within a single city.
How hard your tap water is
The water supplier must provide information on the hardness range at least once a year. This obligation is set out in § 9 paragraph 1 WRMG. The information can be found in the annual drinking water analysis and usually also on the supplier's website via a postcode search.
Munich shows how widely the values vary. Stadtwerke München (SWM) state a range of 13.9 to 17.9 °dH for Munich's drinking water. The average is 15.8 °dH, i.e. 2.8 mmol/l (as of January 2026). This makes Munich a hard-water city. Depending on the district, however, a connection in the same city falls into the medium range. A single figure for an entire city remains an approximation.
| Hardness range | Example locations with values in °dH |
|---|---|
| hard | Munich (15.8), Cologne (18), Berlin (14), Nuremberg (14) |
| medium | Stuttgart (13), Leipzig (13.5) |
| soft | Flensburg (8), Hamburg (7), Bremen (7) |
Knowing the exact value is worthwhile in two cases: when dosing detergent and before buying a filter. In both cases, the information from the supplier is sufficient. All other water parameters are covered by a drinking water analysis.
Damage to appliances and pipes
Limescale precipitates wherever water gets hot or evaporates. Appliances with a heating element are hit first: washing machine, dishwasher, kettle, coffee machine, steam iron. A layer of limescale forms on the heating element. It acts as an insulator, so the appliance needs longer and more energy to reach the same temperature. Eventually, it breaks down.
In the building's pipes, limescale narrows the pipe cross-section and reduces the flow rate. Taps and mixer taps become clogged. At some point, a calcified aerator can no longer be unscrewed. Some of this can be dealt with using household remedies, because calcium carbonate reacts to acid. Citric acid and table vinegar remove deposits in accessible places.
A second effect is less well known. Calcified surfaces in pipes and on aerators are rough and offer bacteria more surface to attach to than smooth material. This becomes critical in rarely used pipes in which water stands for hours. This stagnant water is the real risk factor; the degree of hardness itself plays no role.
Is hard water harmful to health
Hard water is harmless to health. Calcium and magnesium are minerals with a clear function: calcium builds bones and teeth, magnesium keeps muscles and nerves working. The Drinking Water Ordinance (TrinkwV) sets no limit value in its Annexes 1 to 3 for either substance or for water hardness. The body excretes any excess.
A misunderstanding rooted in language persists stubbornly. The “calcification” of the arteries, medically known as arteriosclerosis, has nothing to do with limescale in drinking water. It is caused by deposits of fats and calcium salts in the vessel wall. It is promoted by high blood pressure, smoking and elevated blood lipid levels. Hard tap water is not one of the causes.
From a health perspective, there is therefore no reason to soften water. Anyone considering a filter will decide based on appliance protection, taste or other substances in the water. What else is dissolved in tap water is explained in the guide to substances in tap water.
Limescale, coffee and tea
Hard water from 14 °dH changes the taste of hot drinks. Calcium and magnesium bind some of the aroma compounds that are released from coffee and tea during brewing. Tea tastes flatter and forms a film on the surface. Coffee loses acidity and clarity. Very soft water below 8.4 °dH is also unfavourable, because the coffee then lacks body. For filter coffee, the soft to medium ranges are considered a good compromise.
Many households therefore turn to table-top water filters with an ion exchanger. With these jug filters, hygiene is the deciding factor. The water stands in the container, and an exhausted cartridge can become a source of germs. Details can be found in the guide on the risk of germ contamination in jug filters.
Filtering limescale: what works and what doesn't
Effectiveness is determined by pore size, not price. Calcium and magnesium are present in water as dissolved ions and are therefore smaller than any mechanical filter pore. For comparison: the filter medium of Alb Filter cartridges has a pore size of 0.7 micrometres, while dissolved ions are several orders of magnitude smaller. That leaves two options: chemical exchange for other ions or retention by a membrane.
Activated carbon lets limescale pass through. Activated carbon works by adsorption. Organic substances, chlorine and flavour compounds accumulate on its surface, but dissolved mineral ions do not. A pure activated carbon filter improves the taste and reduces the pollutant load, while the water hardness stays the same. That is how the process works, not a weakness.
Ion exchange lowers hardness by several degrees. An ion exchanger replaces each calcium ion with two sodium ions. This process is used in the Active Plus+ cartridge. Its filter material BIOTON lowers hardness by 4 to 6 °dH, binds chlorine as well as taste-affecting substances and pollutants like activated carbon does, and additionally reduces germs. In hard-water regions, this shifts the value down by a whole hardness range.
Ion exchange increases the sodium content. The TrinkwV (German Drinking Water Ordinance) limits sodium to 200 mg/l, and a reduction of 4 to 6 °dH stays well below that. If you follow a low-sodium diet, it is worth asking the manufacturer for the specific value.
Reverse osmosis removes almost all dissolved substances. A membrane retains up to 99%, including calcium and magnesium. The price: mineral-free water, high installation effort and waste water during operation.
Central water softening systems are installed as whole-house water filters at the mains connection and treat all the water. A limescale filter of this type protects the pipes and all connected appliances, not just one tap. They work reliably but require professional installation, maintenance and salt. The result should not be too soft: heavily softened water becomes corrosive and attacks pipe material. For this, the TrinkwV lists the calcite dissolution capacity as a parameter, with a required value of 5 mg/l calcium carbonate.
Physical processes using magnetic or electric fields alter crystal formation. The limescale remains in the water, so the degree of hardness is unchanged. Among experts, their effectiveness is considered controversial.
Determining water hardness yourself
Test strips are sufficient if you want to measure it yourself. After being dipped, they indicate a hardness range: rough, but sufficient for deciding on detergent or a filter. The analysis values from the water supplier, which monitors hardness continuously, are more precise.
Two points are worth considering before any filter decision. First, your own hardness range rather than the regional average: 15.8 °dH in Munich and 7 °dH in Hamburg lead to different answers. Second, the goal. For protecting appliances, hardness is the right starting point. For taste and pollutants, activated carbon is the better route, and the minerals rightly remain in the water.
Related to this topic
Active Plus+ cartridge – lowers water hardness by 4 to 6 °dH
The Active Plus+ filter cartridge filters with BIOTON, a specially developed filter material. It lowers water hardness by 4 to 6 °dH, binds chlorine as well as taste-affecting substances and pollutants like activated carbon does, and additionally reduces germs – right at the faucet or under the sink.
Frequently asked questions
Can you drink hard water?
Yes. Hard water is harmless because calcium and magnesium are minerals. The German Drinking Water Ordinance sets no limit value for water hardness.
When is water considered hard?
Water is considered hard above 2.5 millimoles of calcium carbonate per litre, i.e. above 14 °dH. The medium range lies between 1.5 and 2.5 mmol/l, with soft water below that.
What are the benefits of a whole-house limescale filter?
A limescale filter at the mains connection, also known as a whole-house water filter, treats all the water in the home and thus protects pipes, hot water cylinders and appliances. It requires professional installation, maintenance and salt. For individual taps, a filter cartridge with an ion exchanger is sufficient.
Does an activated carbon filter remove limescale from water?
No. Activated carbon binds organic substances, chlorine and flavour compounds, while dissolved mineral ions pass through it. Lower hardness can be achieved with ion exchangers, reverse osmosis systems and central water softening systems.
Does limescale in water cause hardening of the arteries?
No. Arteriosclerosis is caused by deposits in the vessel wall, promoted by high blood pressure, smoking and raised blood lipid levels. Hard drinking water is not one of the causes.
How do I find out the water hardness where I live?
The water supplier must communicate the hardness range at least once a year, as regulated in § 9 WRMG (German Washing and Cleaning Agents Act). The information can be found in the drinking water analysis and usually on the supplier's website.
Does limescale damage the washing machine?
Limescale forms an insulating layer on the heating element, energy consumption rises and the appliance is put under greater strain. Dosing detergent according to the hardness range and descaling regularly extend its service life.
Does softened water become aggressive towards pipes?
Heavily softened water attacks pipe material. For this, the German Drinking Water Ordinance (TrinkwV) lists the calcite dissolution capacity as a parameter, with a required value of 5 mg/l calcium carbonate. A water softener should therefore not be set to the lowest possible value.
How many degrees of German hardness equal one millimole per litre?
One millimole of calcium carbonate per litre corresponds to around 5.6 °dH. The German Washing and Cleaning Agents Act uses millimoles, while water suppliers and appliance manufacturers mostly use degrees.
