Someone spends three weeks dialing in a new grinder. Burrs aligned, dose to the milligram, distribution tool, WDT, the whole ritual. Then they fill the kettle from the tap and wonder why the cup still tastes flat.
Brew water for coffee is roughly 98% of what ends up in the cup. The other 2% is the coffee itself. That ratio alone should make water the first variable anyone tunes, and it almost never is.
What is actually in your tap water
Tap water is not neutral. It carries dissolved minerals, mostly calcium and magnesium, plus bicarbonate, sometimes chlorine or chloramine for disinfection, and trace amounts of other ions depending on the pipes and the source. The total dissolved solids reading, or TDS, is the rough sum of all of it.
European tap water varies wildly. Amsterdam sits around 150 mg/L CaCO3 hardness. Berlin pushes 300. Paris lands somewhere between 250 and 300. London is harder still in parts. Move 50 kilometers and the water changes character. Coffee made with the exact same beans and the exact same recipe tastes different in each city, and the water mineral content is most of why.
The SCA water standard, in plain terms
The Specialty Coffee Association published a water quality handbook that gives a target zone for brewing. Target TDS for coffee sits at 75 to 250 mg/L. Target hardness lands around 50 to 175 mg/L CaCO3. Alkalinity around 40 mg/L. Inside that window, coffee extracts cleanly and tastes the way the roaster intended.
Most European tap water is too hard at the top end of that range or well past it. Most zero-mineral filtered water sits below it and produces a thin, hollow cup. The SCA water standard exists because the industry needed a shared reference point. The numbers are not arbitrary.
Magnesium and calcium pull different flavors
Magnesium and calcium are both hardness minerals, but they do different work in the cup. Magnesium pulls fruit and acidity forward. It grabs the brighter aromatic compounds during extraction. Calcium pulls body and sweetness. It binds to different molecules and gives weight on the palate.
A water profile heavy on magnesium tends toward sparkle and clarity. A profile heavy on calcium tends toward roundness and chocolate notes. This is why third-wave water sachets and DIY mineral recipes exist. Cafes that compete at championship level often build their water from scratch using distilled water and added minerals.
The bicarbonate buffer and why too much flattens the cup
Alkalinity, mostly bicarbonate, acts as a buffer. It neutralizes acids during coffee extraction. A small amount of bicarbonate stabilizes the cup and stops sourness. Too much, and it neutralizes the bright acids that make a Kenyan or a washed Ethiopian taste alive.
Berlin water, with its very high alkalinity, will flatten a lighter roast every time. The cup turns muddy and the acidity disappears. The bicarbonate buffer is doing exactly what bicarbonate does. It is not a fault of the coffee or the grind.
What to actually do at home
Brita and similar pitcher filters reduce chlorine and soften water modestly. They do not produce a precise mineral profile, but in soft-water cities like Amsterdam they bring tap water into a workable range. In Berlin or Paris they help less than people think.
Peak Water and similar ion-exchange pitchers let you blend hard and soft water to hit a target hardness. More control, more cost. Third-wave-water style sachets dissolve into distilled water and produce a known specialty coffee water profile every time. Cheap, reliable, slightly fiddly.
Bottled water is the easy answer if the math works for the volume. Volvic sits around 60 mg/L TDS, on the low end but workable. Evian is 300+ and too hard for coffee.
Want a coffee worth the water?
Tuning brew water is half the equation. The other half is the beans. Santo Café roasts single-origin coffee from Mexico in small batches, the kind of coffee that rewards the effort of getting your water dialed in.