Methodology and sources

Every figure on this site links to the document it came from: the water supplier's quality report, a regulator's register or a national open-data file. We never use figures from filter sellers or blogs.

We store hardness as mg/L calcium carbonate and convert for display: 1 °dH = 17.85 mg/L, 1 °fH = 10 mg/L, 1 gpg = 17.12 mg/L, 1 mmol/L = 100.09 mg/L, 1 °Clark = 14.25 mg/L. When a supplier publishes only calcium and magnesium we compute hardness as 2.497·Ca + 4.118·Mg.

Bands follow the USGS/WHO convention: soft < 60, moderately hard 60–120, hard 120–180, very hard > 180 mg/L. Where a country uses its own legal bands (Germany, France) we show them too.

Cities with several supply zones show the median and the range. 'Regional estimate' means the figure is a regional average, not a measurement in that city, and is flagged on the page.

Postcode lookup uses GeoNames postal-code coordinates (CC BY 4.0) and returns the nearest city for which we have data; it is an approximation, not a zone map.

Report an outdated figure by email and we will re-check the source.

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