Four weighted components, out of 100
Every water zone receives a single score from 0 to 100. That headline number is the weighted sum of four components, each measuring a different aspect of water quality. Safety carries the most weight because it covers the parameters with the clearest links to human health.
Safety
Contaminants with direct health significance — including lead, nitrate, PFAS, and microbiological indicators. The single largest driver of the score.
Purity
Disinfection by-products and chemical residues such as trihalomethanes (THMs), bromate, and pesticides — markers of how clean the treated water is.
Compliance
How the water company performed against regulatory standards in the DWI's reporting — the track record of meeting legal limits.
Minerals
Hardness and mineral balance (calcium, magnesium, sodium). Largely an aesthetic and appliance consideration rather than a health one — hence the lowest weight.
Score bands
To make scores easy to interpret, every result falls into one of four bands.
Why most postcodes score Fair
A zone's score is more than a simple average. Certain high-significance parameters apply a hard ceiling — an enforcement cap — that limits the maximum achievable score regardless of how well the zone does elsewhere. This is a deliberately conservative design choice: a serious risk on one parameter should never be averaged away by good results on the rest.
The PFAS cap explains the national picture
PFAS — so-called "forever chemicals" — are an emerging risk where the science is still developing and regulatory limits are tightening. Where a zone's data triggers the PFAS enforcement cap, its score is held to a maximum of 59 — the top of the Fair band.
Because PFAS are now detected so widely across the UK, this single cap is the reason roughly 79% of postcodes score Fair rather than Good or Excellent. It is not a sign that the water is unsafe to drink — it reflects our conservative stance on a risk regulators are still catching up with.
Other parameters apply their own caps, each reflecting the level of concern that parameter warrants:
PFAS cap
Caps the score at the Fair maximum. Reflects our conservative position on forever chemicals as an emerging, tightening risk.
Lead cap
Where lead data warrants it, the score is held to a maximum of 72 — keeping it within the Good band but no higher, given lead's clear health significance.
Microbiological cap
Microbiological indicators (such as E. coli and coliforms) apply a cap, because a microbiological failure is an immediate health concern that should override otherwise good results.
23 parameters across five categories
Each score is calculated from 23 individual parameters, grouped into five categories. Every parameter is drawn from the official DWI compliance dataset for that water zone.
Safety health-significant contaminants
Purity disinfection by-products & residues
Minerals hardness & mineral balance
Aesthetic taste, appearance & odour
Compliance regulatory track record
Official regulatory data, refreshed annually
We don't run our own lab tests or model estimates. Every figure behind a MyTapWater score is taken directly from the body that regulates UK drinking water.
Source
The Drinking Water Inspectorate (DWI) Annual Compliance Reports — the official record of water quality testing for England and Wales.
Refresh cadence
Updated annually when the DWI publishes new compliance data, typically in the first half of each calendar year. Each zone clearly shows its data year.
Coverage
Every major UK water company and over 1.8 million postcodes, mapped to their specific supply zone.
How we keep scoring honest
The methodology is governed by four principles that apply to every score we publish.
- Regulatory data onlyScores are built exclusively from official DWI compliance data — never from estimates, modelling, or self-reported figures.
- No commercial influenceNo water company, filter brand, or advertiser can pay to change a score. Scoring is fully independent of any commercial relationship.
- Transparent & reproducibleThe weights, bands, and caps are published here in full. Anyone with the same DWI data can follow the method and arrive at the same result.
- Conservative on emerging risksWhere the science is still developing — as with PFAS — we apply enforcement caps and err toward caution rather than waiting for limits to catch up.