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Water hardness calculator for coffee

Does your water hit the sweet spot?

Your coffee is 98 percent water. Enter the three values found on every bottle label and in every drinking water report, and see where you land on the chart.

Where to find the values: Every mineral water bottle shows the analysis in the small print. For tap water, every Swiss municipality publishes a drinking water report, usually on the water utility’s website.
Your water
Total hardness (GH)–
Carbonate hardness (KH)–
Total hardness in ppm–
Carbonate hardness KH in °dH Total hardness GH in °dH Your water
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How to read the chart

Two values matter.

Remember: two hardness values matter for coffee. They affect the coffee in different ways, so labels like "medium hard" are too rough.

Total hardness, the vertical axis

This is the sum of calcium and magnesium. These minerals are the solvents: they bind flavour compounds from the ground coffee and carry them into the cup. Magnesium is the more interesting part, because it is especially good at bringing out fruity and sweet notes.

Too little of them, and the coffee tastes thin and flat. Distilled water makes the worst coffee you can brew.

Carbonate hardness, the horizontal axis

This is the bicarbonate. It buffers acids. A little of it is good, because it rounds off sharp edges. Too much, and it neutralises exactly the fruit acids you bought the specialty coffee for. The coffee then tastes flat and chalky.

Rule of thumb: total hardness builds the body, carbonate hardness dampens the fruit aromas. If your coffee tastes flat, look at the bicarbonate value first.

Why two target ranges?

Filter coffee can be softer. There you want maximum clarity and acidity, so as little buffer as possible. For espresso a little more carbonate hardness makes sense: it rounds off the higher concentration and protects the machine, because extremely soft water can attack metal parts.

That is why the two rectangles in the chart sit side by side rather than one above the other. If your point lands in the overlap, you have water that works for both.

Where the target ranges come from

They are based on the Water Standard of the Specialty Coffee Association, split by brewing method. They are guide ranges, not limits: water just outside them won't make bad coffee, it just isn't the optimum. If you want to know exactly, there's no way around trying it yourself.

Conversion, in case you have other units

  • 1 °dH total hardness = 7.14 mg/l calcium or 4.34 mg/l magnesium
  • 1 °dH carbonate hardness = 21.8 mg/l bicarbonate
  • 1 °dH = 17.8 mg/l CaCO₃ (ppm)
  • 1 °fH (French) = 0.56 °dH

The target ranges are guidelines based on the SCA Water Standard and common practice in the specialty coffee scene. They do not replace a drinking water analysis. For questions about drinking water quality, contact your water utility.