Hard water is one of the most common water-quality terms people encounter after moving to a new city, and also one of the most misunderstood. Most people don't think about their water until something changes: you move somewhere new, your hair starts feeling different, or white marks appear on the shower glass and never quite disappear. That is usually the moment a term you'd never paid much attention to suddenly matters.
Hard Water Is Not Dirty Water
One of the biggest misconceptions is that hard water means poor-quality water. It doesn't. Hard water simply contains higher levels of naturally occurring minerals, mainly calcium and magnesium, which the water picks up as it moves through rock and soil. In many parts of Europe this is completely normal: the tap water in London, Paris, Berlin and Rome is hard. Hardness describes what is dissolved in the water, not contamination.
What Makes Water Hard?
As water moves through the ground, it dissolves minerals from rock and soil. The more calcium and magnesium it picks up, the harder the water becomes. Hardness is commonly measured as milligrams per litre of calcium carbonate equivalent (mg/L CaCO₃), and the UK Drinking Water Inspectorate classifies it like this:
| Classification | Hardness |
|---|---|
| Soft | up to 100 mg/L |
| Slightly hard | 100–150 mg/L |
| Moderately hard | 150–200 mg/L |
| Hard | 200–300 mg/L |
| Very hard | above 300 mg/L |
Hardness classification based on calcium carbonate (CaCO₃) concentration. Reference: UK Drinking Water Inspectorate, Water Hardness – Consumer Guidance. Other countries use their own units, such as German degrees (°dH) or French degrees (°f).
Not All Water Is the Same
The difference between cities can be substantial.
