TempoLife › Features › Mineral water mineral content › hardness
Water hardness explained
Hardness is dissolved calcium and magnesium, expressed as if all of it were calcium carbonate. Here is the conversion between millimoles, German degrees, French degrees and parts per million, the classification bands, and the hardness of every water in the database computed from its own analysis.
What hardness actually is
Hardness is not a substance. It is a way of adding up the divalent metal ions in water — in practice calcium and magnesium, because everything else is present in trace amounts — and reporting the sum as though it were all calcium carbonate. That convention is centuries old and it exists because the original test was a soap test: hard water precipitated soap into scum, and how much soap it took to make a lather was the measurement.
The chemistry that produces it is simple. Rain picks up carbon dioxide, becomes weakly acidic, and dissolves limestone and dolomite as it moves through the ground, carrying calcium and magnesium away as bicarbonates. Water that has travelled through granite, basalt or sandstone has nothing much to dissolve and comes out soft. That is why Volvic, filtered through Auvergne lava, is soft and Contrex, from the Vosges limestone, is not.
The units, converted
Four units are in circulation and they are all the same quantity in different clothes. Start from millimoles per litre, which is the chemically honest one:
mmol/l = calcium ÷ 40.08 + magnesium ÷ 24.31, both in mg per litre.
| Unit | One millimole per litre is | Used by |
|---|---|---|
| Millimoles per litre (mmol/l) | 1.00 | chemistry, German consumer bands, water utilities |
| German degrees (°dH) | 5.61 | Germany, Austria, the Nordics and Baltics; appliance settings |
| French degrees (°fH) | 10.01 | France, Belgium, Spain |
| mg/l as calcium carbonate (ppm) | 100.09 | international literature, UK and US utility reports |
| English or Clark degrees (°e) | 7.02 | older UK sources |
So a water at 17.2 °dH is 3.07 mmol/l, 30.7 °fH and about 307 mg per litre as calcium carbonate. Those are all one water; only the convention changed.
The two classifications you will meet
| Band | mg/l as CaCO₃ | mmol/l | °dH |
|---|---|---|---|
| Soft | under 60 | under 0.60 | under 3.4 |
| Moderately hard | 60–120 | 0.60–1.20 | 3.4–6.7 |
| Hard | 120–180 | 1.20–1.80 | 6.7–10.1 |
| Very hard | over 180 | over 1.80 | over 10.1 |
The German scale printed on detergent boxes and built into dishwasher settings uses three coarser bands: weich below 1.5 mmol/l (8.4 °dH), mittel from 1.5 to 2.5 mmol/l (8.4–14 °dH), and hart above 2.5 mmol/l (14 °dH). If your dishwasher asks for a hardness setting, this is the scale it means, and your water utility publishes the number for your street.
Every water in the database, by hardness
| Water | Ca mg/l | Mg mg/l | mmol/l | °dH | mg/l CaCO₃ | Band |
|---|---|---|---|---|---|---|
| Donat Mg | 380 | 1 000 | 50.63 | 283.9 | 5 066.9 | very hard |
| Hepar | 555 | 119 | 18.74 | 105.1 | 1 876 | very hard |
| Contrex | 468 | 74.5 | 14.74 | 82.7 | 1 475.5 | very hard |
| Gerolsteiner Sprudel | 348 | 108 | 13.13 | 73.6 | 1 313.8 | very hard |
| Ferrarelle | 365 | 19 | 9.89 | 55.5 | 989.8 | very hard |
| Badoit | 190 | 85 | 8.24 | 46.2 | 824.5 | very hard |
| S.Pellegrino | 208 | 55.9 | 7.49 | 42 | 749.6 | very hard |
| Apollinaris | 90 | 120 | 7.18 | 40.3 | 718.9 | very hard |
| Vittel | 240 | 20 | 6.81 | 38.2 | 681.7 | very hard |
| Staatlich Fachingen | 96 | 46 | 4.29 | 24 | 429.2 | very hard |
| Perrier | 147.3 | 3.4 | 3.82 | 21.4 | 381.9 | very hard |
| evian | 80 | 26 | 3.07 | 17.2 | 306.9 | very hard |
| Buxton | 55 | 19 | 2.15 | 12.1 | 215.6 | very hard |
| Vichy Catalan | 60 | 9 | 1.87 | 10.5 | 186.9 | very hard |
| Highland Spring | 35 | 8.5 | 1.22 | 6.9 | 122.4 | hard |
| Acqua Panna | 32.6 | 6.8 | 1.09 | 6.1 | 109.4 | moderately hard |
| Fiji Water | 18 | 15 | 1.07 | 6 | 106.7 | moderately hard |
| Volvic | 11.5 | 8 | 0.62 | 3.5 | 61.7 | moderately hard |
| Voss | 5 | 1 | 0.17 | 0.9 | 16.6 | soft |
Waters whose producer publishes a mineralisation bracket rather than individual ions are not in this table: hardness cannot be computed without calcium and magnesium separately.
What hardness does and does not do
It does furring. Heating hard water breaks calcium bicarbonate down into calcium carbonate, which is far less soluble and comes out of solution onto the hottest surface available — a kettle element, a boiler coil, a coffee machine's heat exchanger. That is a maintenance cost and, in espresso machines, a service interval. It also does laundry: hard water binds soap into insoluble scum, which is why detergent doses and dishwasher salt settings are indexed to hardness.
What it does not do is make water unsafe. No health-based guideline value has been set for hardness in drinking water, and both of its components are nutrients that count towards intake — a litre of a hard water is a real, if modest, contribution to calcium and magnesium. The old epidemiology associating soft-water areas with higher cardiovascular mortality has never been resolved into a causal finding, and it is not the basis of any current recommendation.
Two practical caveats. Softeners that work by ion exchange trade calcium and magnesium for sodium, so a heavily softened supply is a sodium question rather than a hardness one — worth knowing if you have been asked to keep salt down. And hardness tells you nothing about the things that actually make water unsafe: nitrate, lead, bacteria. Those are the parameters your utility tests for.
Analyses: the figures each producer publishes on the label or in its technical sheet — see the basis line on every water page. Reference intakes: EFSA Dietary Reference Values. Label claims: Directive 2009/54/EC, Annex III. Mineral-water constituent limits and the fluoride labelling rule: Directive 2003/40/EC. Tap-water parametric values: Directive (EU) 2020/2184. Checked 2026-09-06.
Frequently asked questions
How do I convert °dH to mg/l of calcium carbonate?
Multiply by 17.85. One German degree is 10 mg per litre of calcium oxide, which is 17.85 mg per litre expressed as calcium carbonate. Going the other way, divide by 17.85. One millimole per litre is 5.61 °dH, 10.0 °fH and 100.1 mg per litre as calcium carbonate.
What counts as hard water?
On the classification used in the international drinking-water literature: soft below 60 mg per litre as calcium carbonate, moderately hard 60 to 120, hard 120 to 180, very hard above 180. The German scale used on detergent packets works in millimoles: soft below 1.5, medium 1.5 to 2.5, hard above 2.5.
Is hard water bad for your health?
No health-based guideline value has been set for hardness. The calcium and magnesium in hard water are nutrients and count towards intake. Studies linking soft water areas with cardiovascular mortality are ecological and have never settled the question of cause.
Does hard water cause limescale?
Yes, and that is its real cost. When hard water is heated, calcium bicarbonate breaks down and deposits calcium carbonate on the element — kettles, coffee machines, boilers and shower heads. The scale is the calcium you can see; the water is not damaged by having it.
All waters compared
The full analysis table, grouped by mineral content.
Low-sodium water
The 20 mg per litre label rule, and how much sodium a litre of each water really carries.
Water highest in magnesium
Every water ranked by magnesium, against the EFSA adequate intake.
Water highest in calcium
Calcium per litre ranked, and how much of a day it covers.
Bicarbonate and alkaline water
What bicarbonate is, which waters carry a lot of it, and what it comes packaged with.
Fluoride in bottled water
The EU limit, the under-sevens labelling rule, and where natural fluoride comes from.
One less thing to guess at
TempoLife keeps hydration next to everything else you track, so the day you drank two litres and the day you drank none are both on the record.