Three ways, from most to least reliable.
1. Ask your water company
Every utility publishes hardness by postcode or supply zone, usually on its website within a couple of clicks. It is free, it is accurate for your actual supply, and it takes two minutes.
Figures come in three units and converting is straightforward:
- ppm or mg/l CaCO₃ — the most common
- °dH (German degrees) — multiply by 17.8 to get ppm
- °fH (French degrees) — multiply by 10 to get ppm
2. A test strip
Coffee machines often ship with one, and packs cost very little. Dip, wait, compare to the chart. Accurate enough to place you in a band, which is all you need.
Useful if you are on a private supply, or if you want to check filtered water against unfiltered.
3. Look around the house
Not a measurement, and a good indicator. Hard water leaves white deposits on taps and shower screens, kettles fur up inside within weeks, and soap lathers poorly. If you descale your kettle regularly, your coffee machine needs the same attention.
What the numbers mean for coffee
- Above ~150 ppm (8.4 °dH) — hard. Filter, and descale on schedule. Scale is a genuine threat.
- 50 to 150 ppm — the useful range. Descale on the machine’s prompt and enjoy your coffee.
- Below ~50 ppm — soft. Scale is not your problem; weak, hollow extraction might be.
Then act on it
The point of knowing is choosing correctly. Hard water means a filter is not an upgrade but a requirement. Soft water means a filter makes things worse, and mineral recipes are where the improvement lies.
Questions people actually ask
Does hardness change through the year?
It can, particularly on supplies that mix sources seasonally. Utilities publish a range for this reason. Treat the upper figure as your working number.
Does a water softener help?
Not for coffee. Domestic softeners exchange calcium for sodium, which prevents scale and leaves water that extracts poorly and tastes salty. Use an unsoftened tap for the coffee machine if you have one.
Last reviewed 8 August 2026