Data Methodology & Testing

Baseline figures come from published municipal water quality reports. Under the EU Drinking Water Directive (2020/2184) — and equivalent regulations in the UK, Ireland, United States and Canada — water utilities are required to publish routine biochemical analyses.

• Figures are compiled from the reports published by each city's water utility, which is named on every city page.
• Where a utility publishes a public report we can link to directly, that link appears on the city page so you can check the figure at source.
• Values represent typical measured hardness, not a live reading. Municipal supplies vary seasonally, and we note that range where it is known.

Because Europe lacks a single unified metric for water hardness, municipalities publish data in German Degrees (°dH), French Degrees (°fH), English Degrees (°e), or millimoles per liter (mmol/L). Our proprietary algorithm converts all localized metrics into a standardized milligrams per liter (mg/L) of Calcium Carbonate (CaCO₃) equivalent to allow for accurate cross-border comparisons.

For smaller towns lacking published municipal data, we utilize a geospatial estimation model. By analyzing the primary water source (e.g., groundwater aquifer vs. surface river) and mapping it against known data points in neighboring municipalities sharing the same hydro-geological basin, we project an estimated mg/L value with a ±10% margin of error.