Water for coffee: the variable everyone ignores
Coffee is 98% water. The mineral content of what you pour through your beans matters more than almost anything else. A guide to fixing it at home.
A cup of brewed coffee is around 98 percent water. The grind, the bean, the recipe and the technique all matter — but the largest single ingredient in your cup is the thing you have probably never thought about. Most home brewers use tap water or whatever is in the kettle. In most cities, that water is the reason a careful pour-over still tastes flat.
What you are looking for
Coffee water has two properties that matter: total mineral content (TDS) and the balance of calcium-and-magnesium to bicarbonate. The Specialty Coffee Association's recommendation is around 150 milligrams per litre of TDS, with a bicarbonate level of 40 milligrams per litre. You do not need to memorise these numbers. The takeaway is that pure distilled water makes flat, lifeless coffee — minerals are what bind flavour compounds in extraction — and overly hard, chalky tap water makes dull, scaled coffee with a heavy bicarbonate edge.
What is wrong with your tap water
It depends entirely on where you live. London, much of southern England, Madrid and a lot of the Mediterranean coast have hard water with a high bicarbonate level — coffee made with it tastes dull and slightly chalky, and your machine scales quickly. Berlin, parts of Scandinavia and Manchester have softer water that brews better but still varies. Many North American cities have heavily chlorinated water that puts an unmistakable swimming-pool note into the cup.
The cheapest first step is a basic carbon filter — a Brita pitcher, an under-sink filter, anything that strips chlorine. That alone often improves the cup noticeably.
Going further
If you live somewhere hard and you want to take the next step, the easiest path is to buy bottled water with the right mineral profile. Volvic is a classic café choice — low mineral content, balanced. Some specialty drinkers go further and mix their own "third wave water" from sachets of magnesium and bicarbonate added to distilled or RO water; that is fiddly but produces results that are reliably excellent and identical every time.
Avoid sparkling water (obviously), avoid very high-mineral mineral waters (San Pellegrino, anything labelled as "mineral-rich"), and avoid pure distilled — it brews flat and is not safe for most espresso machines because it is corrosive.
Is it worth the trouble?
If you are spending £20 a bag on single-origin beans, drinking them with tap water is like watching a 4K film on a phone speaker. The information is there; you just are not hearing it. Try the same bean brewed once with your tap water and once with a bottle of low-mineral spring water. The difference is not subtle. After that experiment, you can decide for yourself whether the upgrade is worth the small ongoing hassle. Most people who run the test do not go back.