Your water supplier publishes the hardness for your area, and that is the figure every product in this family is specified against. It is free, it is exact for your network, and a test strip only confirms it.
Where to look
The supplier’s website, usually searchable by postcode or commune, or the annual water quality report they are required to publish. Both give a hardness for the supply you actually receive.
The units, and why there are three
°dH (German degrees) is standard in Germany and Austria. °f (French degrees) is used in France and Italy. mg/l or ppm of calcium carbonate appears in English-language specifications. They convert directly: 1 °dH is about 1.79 °f and about 17.8 mg/l.
What the number means in practice
Below about 7 °dH water is soft and scale is not a problem. Above about 14 °dH it is hard, and it is the range where kettles fur, screens spot and softeners get bought. Between the two, it depends on how much it annoys you.
Why it has to come first
A softener’s capacity is quoted as a volume of water at a given hardness. The same unit treats twice as much soft water as hard water between regenerations, so the specification cannot be read without your figure.
This does not replace the manufacturer's own instructions. Every dimension, thread size, flow rate, operating pressure, water seal depth and load figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A flow rate is reported with the pressure the manufacturer measured it at, and is never converted to another pressure, because no conversion exists. And the second statement repeated rather than assumed: a water softener treats limescale and is not a health product — softened water carries more sodium, not less. A filter reduces what its manufacturer declares it reduces; no benefit for skin, hair or health is claimed here, and drinking water quality is a matter for your supplier.
Questions people actually ask
Are test strips accurate enough?
They are good enough to place you in a band, which is all the sizing needs. Where they are genuinely useful is checking a softener is still working, rather than establishing the supply figure your supplier already publishes.
Does hardness change through the year?
It can, where a supplier blends sources seasonally. The published figure is usually a range or an average for that reason, and sizing against the harder end is the cautious choice.
Last reviewed 5 September 2026