Water Chemistry Simulator: Hardness, Alkalinity & Limescale

Coffee is over 98% water — and its minerals help decide how it tastes. Here, play around with how total hardness and alkalinity change extraction, vibrancy (acidity), and the risk of scale buildup — and where the SCA sweet spot lies.

Hardness and Alkalinity — Two Levers

Total hardness (dissolved calcium and, especially, magnesium) drives extraction: These cations bind to the aroma compounds in coffee and extract them more effectively — magnesium-rich water extracts the most strongly. Alkalinity (bicarbonate) acts as a buffer: It neutralizes acids. A little bit of it provides balance; too much makes the coffee flat and dull, too little makes it taste sharp/sour — and can even make the water corrosive. When high hardness and high alkalinity meet heat, scale forms, clogging up the machine.

The SCA standard recommends a total hardness of approximately 50–120 ppm and an alkalinity of around 40 ppm (as CaCO₃) as a good all-rounder. However, there is no perfect water for everyone: light, fruity coffees prefer softer water with little buffer, while darker roasts benefit from a bit more.

Sources (scientific): Hendon, Colonna-Dashwood & Colonna-Dashwood (2014), "The Role of Dissolved Cations in Coffee Extraction", J. Agric. Food Chem.; SCA / SCAE Water Quality Standard (hardness and alkalinity recommendations).

What You Can Do

With a water filter, you reduce hardness and alkalinity (less scale). If your water is too soft or you want to build it up specifically, mineralizing drops from Apax Lab — such as the Water Mineralizing Drops or the Box Set — can help you adjust hardness and alkalinity precisely. The Apax Lab TDS Meter measures your water's current state, and our page Compose Your Own Coffee Water shows you how to build it from scratch. No matter how good your water is: regular descaling keeps your machine healthy.

Frequently Asked Questions

Which water is best for coffee?

There's no perfect water for everyone. A good all-rounder is within the SCA window: hardness around 50–120 ppm, alkalinity around 40 ppm (as CaCO₃). Magnesium-rich water tends to extract more; softer, lightly buffered water emphasizes acidity and fruit.

What's the difference between GH and KH?

GH (General Hardness) is the calcium and magnesium content — it drives extraction. KH (Carbonate Hardness/Alkalinity) is the bicarbonate content — it buffers acids. Both together plus heat determine the risk of scale.

Why does my machine scale up?

Scale (calcium carbonate) precipitates when hard, alkaline water is heated. The higher the hardness and alkalinity, the faster it occurs. Filtering reduces both; however, you should still descale regularly.

Does a water filter help?

Yes — a filter reduces hardness and alkalinity, and thus the risk of scale. If the result is too soft (sharp/corrosive), you can specifically add some minerals back.

All interactive tools can be found bundled in our Coffee Tools.