How we score
Every WaterGraph test ends in one number, the Home Water Exposure Score, and a verdict you can act on.
The score runs from 0 to 100. It is built from eight parameters, weighted by how much each matters for health, and measured against the UK and World Health Organization drinking-water standards. The method is independent, versioned and open to review.
Three verdicts, one number behind each
Eight readings collapse into a single, comparable score out of 100, with lead reported alongside it. The score sets the verdict on your report.
Clean, drinkable water. Nothing over a regulatory limit.
Something worth knowing about, for taste, appliances or cost. No health risk at the current readings.
Something to fix. A regulatory limit has been exceeded, or lead has been detected.
For audit and technical review, every score also carries a five-band classification (A · 90–100, B · 80–89, C · 65–79, D · 40–64, F · 0–39) and the version of the method it was scored under.
Eight parameters, each one specific
Each is a measurable, regulated indicator of drinking-water quality. We test the eight that matter most for what a household is exposed to, and score them against the current UK limits.
Measured on every test and reported in micrograms per litre. Not weighted into the score: any detectable lead caps it instead (see below). The UK limit tightens from 10 to 5 µg/L by 2036.
Agricultural run-off indicator. A particular concern for infants and in rural areas.
Picked up from copper pipework, especially where water stands overnight. Taste first and, at higher levels, stomach upset.
Watched where fluoridation is active or levels are naturally high in groundwater.
Old mains and internal pipework. Discolouration and taste come well before any health concern.
Affects how much metal leaches from pipes. Low pH increases lead and copper dissolution.
Affects appliances, taste and how soap performs. A baseline for comparing your area.
Buffering capacity. Affects corrosion control and how stable the water is.
This is the current set, version 1.0. Earlier versions tested up to 13 parameters, including bacteria, which left the panel in 1.0. Future versions may add microplastics, PFAS and oestrogen as the lab methods mature. A score always stays valid under the version it was calculated with.
Several of these parameters also appear area by area on the UK water quality map, where you can see how your postcode performs before you test.
Weighted, normalised and capped
Each raw reading becomes a sub-score out of 100. The sub-scores are then combined across three tiers, weighted by health significance.
- Nitrate
- Copper
- Fluoride
- pH
- Iron
- Total hardness
- Total alkalinity
Parameters with a regulatory limit use a piecewise-linear curve: 100 at zero, 60 at the limit, 0 at five times the limit. pH peaks at 100 near 8 and falls away outside the 6.5–9.5 range. Hardness and alkalinity, which have no health limit, score against the UK population median. Lead is measured and reported but has no sub-score of its own.
The exact weights within each tier are part of the versioned methodology and are available to academic reviewers on request.
If lead is detected at any level, the score is capped at 40, Action Needed, for as long as it remains.
Lead cannot be averaged away by good readings elsewhere, so it sits outside the weighted score. The reading is reported as measured, and its presence caps the score until a retest shows it gone. This lead gate is part of the version 1.0 methodology.
Independent, ranked and reasoned
Every report includes ranked recommendations: what to fix, what to keep an eye on and what to skip. Each one is tied to a specific parameter and to whatever is driving your score.
Our recommendations come from independent research, not commercial relationships.
We don’t sell filters. Every recommendation is chosen on your readings and the evidence for what works; no brand or retailer has a say in it. If we ever earn anything from a link, we say so on the page.
When we suggest a specific product, treatment or supplier, we cite the study or regulatory data behind it. Recommendations are ranked by the expected effect on your score and by how directly they address the flagged parameter.
What we measure against
We benchmark against the Drinking Water Inspectorate (DWI), the body that sets the UK’s statutory limits. The DWI’s reduction of the lead limit from 10 to 5 µg/L by 2036 is reflected in our scoring.
Where UK guidance is silent or conservative, we refer to the World Health Organization Guidelines for Drinking-water Quality.
The method is open to review. The calculation logic, the normalisation curves and the lead gate are described on this page, and the weights are available to academic reviewers on request. Any score can be reproduced from its raw readings.
The method is versioned. A score stays valid under the version it was calculated with; we do not re-score old results when the method changes.
A score, not a diagnosis
The Home Water Exposure Score is context. It tells you where your household’s water sits against the regulatory limits and the readings around you.
It is not a medical diagnosis. It does not certify your water as safe or unsafe, and it does not replace the statutory testing done by your water company or the Drinking Water Inspectorate.
We provide measurement, not medical advice. For regulatory-grade confirmation, an accredited laboratory test is the next step.
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A lab test of your tap water, your Water Health Score and a Care Plan, posted UK-wide. From £49.
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