Water hardness in Spain: how it is measured and what you have at home

Water hardness measures how much calcium and magnesium the water carries in solution. It is the single figure you need in order to know whether your installation will scale up, and how fast. In Spain it is almost always expressed in French degrees (°f), where 1 °f equals 10 milligrams of calcium carbonate per litre. Below 15 °f the water is soft and scaling is slow; above 35 °f it is very hard and an untreated hot water system degrades within a few years.

The practical problem is that the same figure is published in four different units depending on who measured it, and that the variation across Spain is enormous: some cities sit below 8 °f and others above 55 °f. This article settles both.

What hardness actually measures

Total hardness is the sum of the calcium (Ca²⁺) and magnesium (Mg²⁺) ions present in the water, conventionally expressed as if it were all calcium carbonate. That is why every unit ends up referring back to mg/L of CaCO₃, even though the water also contains magnesium.

Within total hardness there are two components. Temporary or carbonate hardness is the fraction that precipitates when water is heated: it is what causes scaling. Permanent hardness, tied to sulphates and chlorides, does not precipitate with heat. In most Spanish mains water the dominant fraction is the temporary one, which is precisely the troublesome one.

The units and how they convert

If your municipal report is in mg/L and your dishwasher manual talks in German degrees, this table settles the conversion.

The scale: from soft to very hard

The practical threshold sits around 25 °f. Below it, a well-sized installation copes reasonably without treatment. Above it, the question is no longer whether scaling will happen but when it will become noticeable, and that depends mostly on the temperature the domestic hot water runs at.

Water hardness by region in Spain

Geology decides. Granite and slate regions (Galicia, much of the Madrid sierra, the whole northern coast) give soft water. Limestone and gypsum sedimentary basins (the Mediterranean arc, the Ebro valley, both Castiles, the Balearics) give hard or very hard water. The table below is indicative: within a single city the value changes with the source feeding each district and with the time of year.

How to find the official figure for your municipality

1. Your water utility's website. All of them publish a water quality report per supply zone, usually with hardness in mg/L of CaCO₃ or in °f.
2. SINAC, the Spanish Ministry of Health's national drinking water information system, which centralises data for supply zones across the country.
3. Your water bill, which in some municipalities states the average value for the period directly.
4. In Portugal, ERSAR's information system and the local operator's website serve the same purpose.

How to measure your own water

1. 01

   Test strips

   The quick option, and enough for a home. Dip for a few seconds in a glass of cold tap water and the colour scale gives you the band. Low cost, accuracy within a few degrees. Enough to know which row of the table you are in.

2. 02

   Drop titration kit

   You add reagent drops to a fixed volume of water until the colour turns, and the number of drops gives the value. Considerably more accurate than a strip, and still a five-minute method with no instruments.

3. 03

   Laboratory analysis

   Necessary when sizing a communal or industrial installation, or when the water does not come from the public mains. It gives total hardness, calcium hardness, alkalinity, pH and conductivity, which is what really allows an informed decision.

Always sample cold water, letting the tap run for half a minute. Hot water has already precipitated part of its carbonate and will read lower than the true value.

The signs that give hard water away without measuring anything

• White marks on the shower screen, the taps and freshly washed glasses.
• Soap struggles to lather and leaves a residue feeling on the skin.
• The shower sprays unevenly because some holes are partly blocked.
• Hot water takes longer to arrive than it did a few years ago with the same boiler.
• The kettle or coffee machine element furs up with white crust within weeks.

You have the number. What do you do with it?

The hardness value drives two decisions: whether to treat, and with what. The first is settled by the scale above. The second depends on what exactly you want to achieve, because the two families of treatment do different things to that number.

The full comparison, with running costs and water consumption, is in Salt softener or physical treatment.