A mug of filter coffee is about ninety-eight percent water, an espresso not far behind, and a pot of tea is basically flavoured water. So the water is not a background detail. Its minerals pull flavour out of the grounds or the leaf, buffer the acidity, and decide whether your machine slowly furs up with scale. Change the water and the same beans can come out sharp, flat, or suddenly right.
What coffee actually wants
The specialty-coffee people have argued this down to numbers. Their target water lands around 150 mg/L of total dissolved solids, with a little calcium and magnesium to carry flavour, a small bicarbonate buffer near 40 mg/L to hold the acidity steady, and a roughly neutral pH. Distilled water, empty of all that, extracts poorly and tastes hollow. Hard, mineral-heavy water goes the other way. It over-extracts, mutes the bright notes, and cakes your kit in scale. The sweet spot is the middle, with just a touch of buffer.
Espresso wants it softer than you think
Espresso adds a catch. A machine with a boiler wants the lower half of that range, nearer 75 to 150 mg/L, because every milligram of calcium and magnesium that passes through hot eventually plates the boiler wall as scale. So the water that pours a lovely cup can quietly be killing the machine. Baristas answer this with one name more than any other, Volvic. It is soft, a TDS of about 110, low enough to spare the boiler, with just enough bicarbonate to keep the shot from turning sour. If you pull shots daily and buy one water, that is the one.
Filter coffee is more forgiving
Drip and pour-over have no boiler to protect, so they take a fuller water, up toward the top of the range. Highland Spring and Buxton sit in that heavier band and brew a rounder cup, and even Acqua Panna, a shade too hard for a boiler, is happy in a filter. The light shelf is the right aisle to browse.
Too soft and too hard both miss
You can overshoot either way. A near-empty water like Voss or Spa Reine, lovely to drink, gives the coffee almost nothing to grip and no buffer to catch the acids, so the cup turns thin and sour. At the far end the mineral heavyweights are a real mistake. Brew with Contrex or any of the full-body crowd and you dull the cup and cake the machine, and even the famous Evian, high in calcium and bicarbonate, is better in the cup than the boiler.
One mineral helps, one hurts
It is worth knowing which mineral does what, because the two hardness minerals pull opposite ways. Calcium is the troublemaker, the one that scales a boiler and, in tea especially, dulls the cup. Magnesium is the quiet ally, lifting extraction and adding a little brightness. So a water that leans magnesium over calcium brews better than its hardness figure alone would suggest, which is the sort of split our cation reading is built to show.
Tea is fussier still, and older about it
Tea has fussed over water longer than coffee has existed. The eighth-century Classic of Tea already ranked mountain-stream water above river water and both above well water, roughly the soft-to-hard order chemistry would draw today. The trouble with hard water is calcium again. It binds the tea’s polyphenols into an insoluble film, the grey scum on a cup of over-hard tea, and strips a good fifteen to thirty percent of those polyphenols out of the drink. High bicarbonate compounds it, lifting the brew’s pH until the colour turns dark and muddy and the brightness dies. It is why a fine tea brewed in chalky London water tastes flat, and why delicate green and white teas, the most sensitive of all, want the softest water you can find, from the bottom of the hardness table, never the top.
The rule of thumb
None of this needs a water report on the counter. For coffee reach for a soft-to-moderate still with a little bicarbonate, and go softer again for espresso to spare the machine. For tea go softer still, low in calcium above all. Keep the mineral giants for the dinner table where they earn their place. To settle your everyday drinking water first and worry about the kettle second, the quiz is the fastest way in.