Which water for tea: hard, filtered or bottled

For tea, the right water is low in limescale, free of any chlorine taste, fresh, and heated once to the right temperature. Depending on where you live, filtered tap water may be enough; in a very hard-water area, a low-mineral bottled water will make a visible difference in the cup.

A cup of tea is a few grams of leaves for a quarter of a litre of water. It makes sense that the water weighs on the result, and this is not just a matter for purists: you can see limescale and you can smell chlorine.

Limescale: the film and the colour

You have probably seen that thin, dull film that forms on the surface of a black tea as it cools, leaving a brown ring on the cup. It is neither dust nor a fault in the tea.

A team from the Swiss Federal Institute of Technology in Zurich (ETH) studied it in 2023. This film is made of organic matter from the tea, with islands of calcium carbonate. The researchers found that it did not form at all with distilled water, that calcium, magnesium and bicarbonates had to be present, and that the film became sturdier the harder the water was. In other words, some of the compounds in the tea bind to the limescale in the water and stay stuck to the film and to the sides of the cup.

Limescale also affects the colour. Thearubigins, the red-brown pigments that colour black tea, change shade with the acidity of the liquid. Hard water, which tends to be alkaline, gives a darker, deeper tea. Conversely, a few drops of lemon lighten the cup by making it more acidic, and citric acid also stops the film from forming. That is why tea with lemon stays clear on the surface.

The same thing happens with coloured infusions. The hibiscus in Chant des Sirènes infusion owes its red to anthocyanins, pigments that are very sensitive to pH: their colour is at its most intense and vivid in acidic conditions, and it changes most towards neutral and alkaline.

Knowing how hard your water is

Water hardness is its calcium and magnesium content. In France it is expressed in French degrees (°f), and the Île-de-France regional health agency (ARS), for example, classifies it as follows:

• under 10 °f: very soft water;
• from 10 to under 20 °f: soft water;
• from 20 to under 35 °f: hard water;
• 35 °f and above: very hard water.

To find out about yours, there are three sources: the annual summary sent with your water bill, the test results posted at the town hall, and the official website eaupotable.sante.gouv.fr, which gives results by municipality. Your kettle also gives a clue: if it furs up within a few weeks, your water is hard.

Chlorine in tap water

Tap water is disinfected, most often with chlorine, and sometimes a taste or smell remains. In a cup of tea, especially a plain tea or a green tea, that taste can be noticeable.

The simplest solution is the one recommended by the French public information website sante.fr: leave the water standing for a while, uncovered, in a jug, in the fridge if you like. The chlorine taste fades. Fill the jug in the evening for the next morning's tea, and the problem is solved without buying anything.

Filter jugs are the other option. Most combine activated carbon, which holds back some of the chlorine and off-tastes, with a resin that reduces some of the limescale. They only do this job properly if the cartridge is changed as often as the manufacturer says: a spent cartridge no longer filters much.

Bottled water: read the dry residue

If your water is very hard or you want to compare, bottled water has one advantage: its composition is printed on the label. The figure to look at is the dry residue at 180 °C, that is, the amount of mineral salts left when a litre of water is evaporated. French regulations set the following labels:

• very low mineral content: dry residue under 50 mg/l;
• low mineral content or oligomineral: dry residue under 500 mg/l;
• rich in mineral salts: dry residue over 1,500 mg/l.

For tea, aim for the bottom of the table: a water with a low dry residue and, if the label says so, low in calcium and bicarbonates, since those are what form the film. Leave aside waters rich in mineral salts.

There is no need, however, to go as far as distilled water. It gets rid of the film, but the aim is water with few minerals, not empty water.

Not reboiling: what we really know

It is advice you read everywhere: use fresh water, do not reboil the water left in the kettle. The usual explanation is that boiling drives off the gases dissolved in the water, oxygen in particular, and that water boiled twice would give a flatter cup.

To be honest: the first boil already drives off most of those gases, and a blind tasting run by the American team at America's Test Kitchen found no difference between water boiled once and reboiled water. We still keep the habit of starting with fresh water, simply because heating just the right amount costs nothing and saves you asking the question.

Above all, many teas do not need boiling water. Our Nuit de Marrakesh green tea is steeped at 70 to 80 °C: let the water cool for a few minutes after boiling, or switch the kettle off before it boils.

Temperature, the other half of the water

The best water in the world will not make up for the wrong temperature. Each of our tea pages gives its own, and they are not alike:

• green tea, such as Nuit de Marrakesh: 70 to 80 °C, 2 to 3 minutes;
• black tea, such as Âme de Sri Lanka: 100 °C, 3 to 5 minutes;
• pu erh, such as Pu Erh d'origine: 100 °C, 4 to 5 minutes;
• fruit infusion, such as Chant des Sirènes: 100 °C, 10 to 12 minutes.

The settings for all our families of teas and infusions, amounts included, are gathered in our guide how to brew loose tea.

If you are wondering what temperature changes to the amount of theine (caffeine) in the cup, we explain it in our article on theine and caffeine.

Frequently asked questions

Why does a film form on my tea?
Because your water contains limescale. The film combines compounds from the tea with calcium carbonate; it does not form with distilled water. Softer water limits it, and a few drops of lemon almost entirely stop it from forming.

Is tap water fine for tea?
Yes, if it is soft and has no strong chlorine taste. To reduce the chlorine, let it stand for a while in an uncovered jug before heating it.

Which bottled water should I choose for tea?
A still water with low or very low mineral content, with a low dry residue on the label. Avoid waters rich in mineral salts.

Is a filter jug enough?
It reduces some of the chlorine and limescale, as long as you change the cartridge regularly. If your water is very hard, compare it with a low-mineral water: the film will tell you which suits best.

Does tea need boiling water?
Not always. Green teas are generally steeped between 70 and 80 °C; black teas, pu erh and fruit infusions at 100 °C. Follow the temperature given on each product page.

Further reading

All our teas and infusions are in our loose teas and infusions collection, each with its amount, temperature and infusion time. Try a simple test: the same tea, brewed side by side with your tap water and with a low-mineral water. You will see the difference before you even taste it.