Where our water data comes from

Water companies publish results for supply zones, reporting wards or other service areas. These are usually summaries of samples taken during a reporting period. They are not measurements at every postcode or every tap.

The sources we use

Published supplier reports provide the measurements. Thames, Affinity, Yorkshire, United Utilities and Severn Trent supply official postcode lookups. Northern Ireland uses NI Water’s annual postcode-to-zone file and hardness tables, published through OpenDataNI under the Open Government Licence. An annual mapping is evidence for that reporting year, not a live guarantee about a changed supply.

Selected Yorkshire annual PDFs, United Utilities rolling 12-month reports and Severn Trent reports are checked weekly. Yorkshire’s total hardness expressed in mg Ca/l is converted to calcium-carbonate equivalent using its official factor of 2.5. We retain the original hardness units and do not confuse this with the separate calcium measurement. A rolling report’s update date is not treated as an exact sampling-period end date.

Scottish Water’s 2016–2024 annual hardness tables retain published calcium, magnesium and supplier-calculated hardness. We preserve exact zone names and separate renamed areas. A source-area history does not establish historical postcode boundaries; sample counts and ranges are unavailable in these tables.

Severn Trent’s dated quality statistics are shown separately from its typical hardness summary, whose sampling period is not stated. French hardness degrees are converted to mg/l as calcium carbonate by multiplying by 10. Undated hardness and rolling periods do not appear as annual history points.

Yorkshire and United Utilities also publish individual samples labelled by statistical area. We retain their complete sample records separately, including actual dates, original units and detection qualifiers. We do not turn those administrative areas into invented water-supply zones. Wessex’s full reports currently require browser collection because hosted downloads are restricted.

Government regulator guidance helps explain measurements and standards. National regulatory statistics and environmental river or bathing-water datasets cannot fill missing drinking-water readings at a postcode. Geographic tools identify places; they do not establish which water pipe serves a home.

Postcodes and places

Our postcode checker uses a supplier-backed match when available. We do not substitute a nearby town when we cannot verify a postcode. Older town pages may show a surrounding supply area; this does not establish which zone supplies every address in that town.

Measurements and missing values

We retain the reported value, unit, period, source, available sample counts and qualifiers. “Less than 0.67” stays a less-than result; it does not become an exact 0.67 reading. Missing hardness is unavailable, not zero or soft water. We do not derive hardness from conductivity without a validated method for that source.

Dates and updates

The reporting period tells you when the measurements relate to. The retrieval date tells you when a source was collected; the source-check date tells you when it was last checked. A newly retrieved report can still contain older measurements. Report histories are being assembled, and coverage is incomplete.

Registered sources are checked daily, with separate weekly structured-dataset collection and agent review for new publications. Recognised formats must pass completeness and context checks before publication. Changed formats and ambiguous results are held for review. The site shows approved source editions, not a claim that suppliers measure or publish every day.

See current supplier coverage and reporting periods. Each result links to its original source. Download our source register and reusable data.

Hardness and comparisons

Hardness is expressed as mg/l of calcium carbonate equivalent. Supplier labels can differ. Our shared categories follow DWI’s hardness guidance: soft up to 100, slightly hard above 100 to 150, moderately hard above 150 to 200, hard above 200 to 300, and very hard above 300 mg/l. Appliance conversions use rounded household factors: divide by 100 for mmol/l, multiply by 0.056 for German degrees, 0.1 for French degrees and 0.07 for Clark degrees.

We compare historical hardness only where the reporting area, metric and units match. Changes in zone boundaries, sampling or methodology can affect comparisons. Two annual values do not establish a sustained trend.

What results can tell you

Hardness describes minerals and is not a health score. A mean below a limit does not mean every sample complied. Historical exceedances need the supplier’s investigation and context; they do not by themselves establish a current incident at your property.

Read the linked original report alongside our summary. If the source differs, tell us about a correction.