How our data works.
Waterdial publishes estimated water-hardness values for cities, converts them into the units used around the world, and turns them into general guidance for household appliances. This page explains where the numbers come from, how we present them and what they cannot tell you.
What water hardness is
Hardness is mainly the amount of dissolved calcium and magnesium in water. It is usually reported as milligrams per litre (mg/L, the same as parts per million, ppm) of calcium carbonate (CaCO₃). Hard water leaves scale on heating elements and glassware and changes how much detergent you need. The World Health Organization does not set a health-based guideline value for hardness: it is an aesthetic and scale issue, not a drinking-safety one.
Where each figure comes from
For every city we look for, in this order:
- The water utility's own published water-quality report, register or hardness page.
- Data published by a government or regulator.
- Peer-reviewed or institutional studies of city-level tap water.
- Where a primary page cannot be read in full, a third-party summary of utility data. Records that rely on this are named as such in their source line (for example "summary of ... data"), and we intend to replace them with the primary report.
Every city page links to its source and shows the date we last checked it. We do not use figures from water-softener or filter retailers, because their numbers are often rounded, outdated or unsourced.
What the numbers mean
- Reference estimate: the value we use for classification and conversion. It is the utility's published citywide figure where one exists; otherwise the midpoint of the published range or the value stated for the main supply zone. Some legacy records do not yet document the calculation or reporting period. Treat those figures as reference estimates, not verified statistical averages; consult the linked supplier before choosing a setting.
- Range: the lowest and highest values in the source for that city or its zones.
- Supply zones: where a source breaks a city down by zone, treatment plant or reservoir, we show those values on the city page, because the tap in one suburb can differ a lot from another.
Confidence labels
| Label | Meaning |
|---|---|
| High | A citywide figure published by the utility or regulator for a recent period. |
| Medium | An estimate: for example the midpoint of a published range, a single sample, a zone-based average, a regional figure, or a summary of utility data rather than the utility's own page. |
Neither label means the figure is exact for your home. Local blending, seasonal changes, private treatment (such as a home softener) and your building's plumbing can all change the result. A test strip or your utility's postcode lookup is always better than a city estimate.
Unit conversions
We convert from mg/L as CaCO₃ using these factors and round to two decimal places:
| Unit | mg/L as CaCO₃ per unit |
|---|---|
| mmol/L (total hardness) | 100.0869 |
| German degree (°dH) | 17.848 |
| French degree (°fH) | 10 |
| Clark degree (°e) | 14.254 |
| Grain per US gallon (gpg) | 17.118 |
Some manuals use slightly different national definitions, so a converted value can differ a little from a figure printed in a manual.
Hardness bands
| Band | mg/L as CaCO₃ |
|---|---|
| Soft | 0–60 |
| Moderately hard | Above 60 to 120 |
| Hard | Above 120 to 180 |
| Very hard | Above 180 |
These follow the widely used USGS-style classification. Other countries and manufacturers use their own scales, so a value described as "soft" here may fall in a different band in your appliance manual.
Appliance guidance
Hardness bands describe the water, not a universal appliance setting or maintenance interval. Match the converted reading to your model’s table. Coffee-machine prompts and manufacturer instructions determine descaling; hardness alone does not. Humidifier care is independent of the hardness band: our portable-humidifier guidance follows the US EPA guidance. See our worked example and reading guide.
Recent editorial changes
20 October 2026: removed universal appliance settings and fixed descaling intervals, separated humidifier care from hardness bands, and added a worked conversion guide. These editorial changes do not constitute a fresh verification of every city dataset.
How often we update
Each record shows the date it was last checked. We aim to review every record at least once a year and sooner when a utility publishes a new report or a reader sends a correction. If a source goes out of date or disappears we say so on the page rather than silently keeping the old figure.
Translations
The site is available in eight languages. English is the reference version. Translations were prepared with automated assistance and may contain errors or awkward wording, so translated city and directory pages are not submitted to search engines. If you notice a mistake, please tell us.
Independence and funding
Waterdial is independently operated and funded by display advertising and, if enabled, affiliate links. Advertising and commercial relationships never change a hardness figure, classification or recommendation. See our affiliate disclosure and privacy policy.
Limitations
- Figures are estimates for cities, not measurements of your tap.
- Only a limited number of cities are covered so far. For anywhere else, enter a test-strip or utility value in the checker.
- This site does not provide plumbing, engineering, health or safety advice.
Corrections
If you find an out-of-date figure, a broken source or an error, send the city, the correct value and a link to the utility's report through the contact page. We check every report against its source before changing a record.