Use your postcode to identify the water supplier, then obtain the current supply-zone result from the supplier itself.
Why a national map is not enough
Maps are useful for orientation, but neighbouring supply zones can have different sources or blends. The Drinking Water Inspectorate recommends asking the water company for an accurate measurement because local variation may not appear on a map.
Three reliable steps
- Use your postcode to identify the company responsible for water supply, not only wastewater.
- Open its water-quality or supply-zone checker and enter the full postcode.
- Record the hardness value and unit, ideally mg/L as calcium carbonate.
Keep the full result. Some reports show calcium, degrees Clark, French degrees or German degrees. Use our converter before comparing a report with equipment specifications.
If a result looks surprising
Check that you selected the clean-water supplier and the correct address. New developments, private supplies and boundary areas need particular care. A home test strip can provide a sense check, but the supplier result should normally be the better starting point for mains water.
Sources and further checks
Sources are provided for verification. Product specifications and local supply information can change, so check the current primary source before acting.