Why Water Is the Most Important Factor in Coffee

Why Water Is the Most Important Factor in Coffee

You invest in freshly roasted beans, a good grinder, the right brewing temperature. But have you ever thought about your water? Filter coffee is 98% water. Espresso is still over 90%. Water isn't just a carrier. It's an active player in extraction. It determines which flavors are pulled from the coffee and which are left behind.

The Role of Minerals

Water is never just H2O. It contains dissolved minerals, mainly calcium and magnesium. And these minerals make the difference. Magnesium is particularly good at binding to the fruity and acidic flavor compounds in coffee. Calcium contributes more to body and mouthfeel. If your water has too few minerals (distilled water or pure reverse osmosis water), it lacks the power to extract flavors from the coffee. The result: flat, hollow, sharp taste. If your water has too many minerals, the coffee turns muddy and bitter, and your machine scales up.

What Is TDS and Why Does It Matter?

TDS stands for Total Dissolved Solids, the total amount of dissolved substances in water, measured in ppm (parts per million). The Specialty Coffee Association (SCA) has established clear recommendations based on decades of research: ideal brewing water has a TDS between 75 and 250 ppm, with a target of around 150 ppm. Add to that a total hardness of 50 to 175 ppm, alkalinity of 40 to 70 ppm, and a pH close to 7.0.

These values aren't academic constructs. They're based on taste tests with thousands of participants. Coffee brewed with water in this range tastes more balanced, clearer, and more complex.

Water in Switzerland

The good news: Swiss tap water is among the cleanest in the world. The less good news: hardness varies significantly by region. In Zurich, it sits around 10-11 degrees German hardness (dH), which is in the medium range and works quite well for coffee. In other regions, especially where water flows through limestone, hardness can be significantly higher. You notice this from the white deposits in your kettle.

If your tap water is very hard (above 20 dH), a simple table water filter can already make a noticeable difference. It reduces limescale and chlorine without completely removing the useful minerals. For getting started, that's perfectly sufficient.

Which Water for Which Method?

Pour over and filter coffee are most sensitive to water quality. Here you want water in the mid TDS range (100 to 200 ppm) so the fine nuances come through. French press handles slightly harder water (150 to 250 ppm), because the longer contact time and absence of a paper filter already create more body. With espresso, water quality matters three times over: it affects flavor, crema, and the lifespan of your machine.

What You Can Do Right Now

Try this experiment: brew the same coffee once with your tap water and once with a still mineral water (like Volvic or Evian, which both sit in a good TDS range). The taste difference will surprise you. Water with too few minerals makes coffee flat and sour. Water with too many makes it dull and bitter. In the sweet spot between, the full character of the bean unfolds.

You don't need lab equipment. A simple TDS meter costs around 15 Swiss francs and shows you in seconds what's in your water. It's one of the cheapest upgrades you can make for your coffee.

At Roestpost

Every week a new coffee from a different Swiss roastery. If you want to get the most out of every bean, it's worth taking a look at your water. Because the best bean deserves the best water. Discover our marketplace and find your next favorite coffee.