Water for coffee and tea: why hardness decides everything

In short: coffee usually calls for water with a TDS of roughly 75 to 150 mg/L and moderate hardness, which pulls the flavour out without leaving limescale. Very soft water gives flat, sour coffee; hard water tastes soapy and quickly furs up the machine. Green tea wants softer water, while black tea can take something a little denser.

What the water does in the cup

Coffee in the cup is almost entirely water, and its minerals decide what gets pulled out of the bean. Magnesium brings out sweetness and fruity notes, calcium gives the cup body but settles as limescale when there is too much of it. Bicarbonates damp down acidity, so a lot of them leaves the cup flat. Distilled water is no good at all, as it has nothing to dissolve the compounds in the bean with and the coffee comes out hollow and sour.

• Magnesium draws sweetness and fruity notes out of the bean
• Calcium gives body, but in excess it turns into limescale
• Bicarbonates damp down acidity, and too many of them make coffee flat
• Distilled water is no good for coffee: there is almost no extraction

What kind of water does coffee need?

For espresso and filter, use water with a mineralisation of roughly 75 to 150 mg/L and moderate hardness: enough flavour, little scale. Mineral water whose salts are counted in grams is wrong for a coffee machine, and Narzan at 2 to 3.5 g/L will give you a harsh taste and rapid scaling. Black tea tolerates denser water and comes out stronger, while green tea and oolong open up on soft water. Check the actual values on the label of the batch, as natural water is given as a range.

• Espresso and filter: TDS of roughly 75 to 150 mg/L, moderate hardness
• Black tea tolerates denser water and the flavour comes out stronger
• Green tea and oolong open up on soft water with low mineralisation
• Check the actual values on the batch label, since the range is natural

Limescale in Phuket

Tap water and well water on the island are often hard, so coffee machines and kettles fur up faster than the manual promises. The sign is simple: white deposits in the kettle after a couple of weeks. On bottled table water with low mineralisation, equipment lasts noticeably longer, although you still descale it, just less often. The Thai standard for water in sealed containers caps hardness at 100 mg/L as calcium carbonate, which counts as soft water.

• Local tap water is often hard, and a coffee machine furs up quickly
• On bottled table water with low TDS, a machine lasts noticeably longer
• Descaling is still needed, just less often
• A kettle with limescale is a sign that this water is not one to make coffee with

What to buy in practice

At home, the easiest thing is to keep an 18.9 L bottle of table water: it covers the coffee machine, the kettle and drinking. A cafe or a restaurant should set up a tasting: brew the same coffee on filtered water and on bottled water with a known composition, and the difference is audible straight away. For tea, choose water with no pronounced mineral tang, or it will override the leaf. Medicinal-table water is not used for brewing.

• Home coffee machine: table water with low mineralisation from an 18.9 L bottle
• Cafe and restaurant: run a tasting comparing filtered water with bottled water of known composition
• For tea: water with no noticeable mineral tang, so it does not override the leaf

Sources

• Thai Ministry of Public Health Notification No. 462 (2025): hardness of water in sealed containers
• GOST R 54316-2020: composition of Narzan water (table A.1)