How We Calculate Water Quality Scores

Overview

Each area on TapWater.uk receives a safety score between 0 and 10. A score of 10 means every measured parameter is well within its regulatory limit; a score of 0 would indicate one or more parameters at or exceeding their legal maximum. In practice, the vast majority of areas in England and Wales score above 7.

The score is not a pass/fail: it is a relative indicator of margin from regulatory limits, weighted by the health significance of each parameter. A higher score means more headroom between measured levels and the limits set by the Drinking Water Inspectorate.

Data Sources

The overall score is derived from two layers of data, combined with fixed weights:

Drinking Water (80% weight) — data from the Drinking Water Inspectorate covering regulated parameters tested at the tap. This is the primary layer because it directly reflects the water you drink. It covers 48 parameters tested on a regular schedule across all supply zones in England and Wales.
Environmental Context (20% weight) — data from the Environment Agency's water quality monitoring network, which measures contaminants in source waters, rivers, and groundwater in the surrounding area. This layer adds broader ecological context and can indicate emerging trends before they reach treatment works.

Drinking water data is weighted at 80% because it is the most direct measure of what comes out of your tap after treatment. Environmental data provides useful context but does not reflect the output of the treatment process.

Scoring Formula

For each measured parameter, we calculate a parameter score based on how close the measured value is to its regulatory limit:

parameter_score = 10 × (1 − measured / limit)

A reading at 10% of its limit gives a parameter score of 9.0. A reading at the limit gives 0. Readings above the limit are clamped to 0 (they cannot make the overall score negative, but they pull it down significantly due to their weighting).

Parameters are grouped into three tiers based on their health significance. Each tier carries a different weight in the composite drinking water score:

Tier 1 — Weight 3.0

Lead, PFAS (per- and polyfluoroalkyl substances), E. coli, total coliform, arsenic. These parameters carry the greatest weight because exceedances pose the most serious acute or chronic health risks.

Tier 2 — Weight 2.0

Nitrate, pesticides, trihalomethanes (THMs), copper. Significant health implications at elevated concentrations, particularly for vulnerable groups.

Tier 3 — Weight 1.0

Turbidity, chlorine, pH, hardness, iron. These parameters affect taste, appearance, and infrastructure, but pose lower direct health risk within regulatory ranges.

The final overall score combines the two layers as follows:

overall_score = (drinking_score × 0.8) + (env_score × 0.2)

Grade Scale

Scores are mapped to a five-band grade for easier interpretation:

Score range	Grade	What it means
9.0 – 10	Excellent	All parameters well within limits; very high margin.
7.0 – 8.9	Good	Parameters comfortably within limits.
5.0 – 6.9	Fair	Some parameters closer to limits; worth monitoring.
3.0 – 4.9	Poor	One or more parameters significantly elevated.
0 – 2.9	Very Poor	Parameters near or at regulatory limits.

Transparency

Every report page shows the individual parameter readings that contribute to the score, so you can see exactly which substances were measured, at what concentration, and how that compares to the regulatory limit. We never present a score in isolation.

Where data for a parameter is missing for a given period or zone, we exclude that parameter from the weighted average rather than treating a missing value as zero. This means the score reflects only what has actually been measured and reported. Where missing data is extensive, we flag this on the report.

Limitations

Our score is a useful summary, but it does not capture everything. Key limitations to be aware of:

Infrastructure age. Lead pipes in older properties can leach lead into water even when the supply zone score is high. DWI data is collected at the treatment works and at a sample of taps — it may not reflect your specific property.
Private water supplies. Properties served by private boreholes or springs are not covered by DWI data and are not included in our scores.
Sampling frequency. Some parameters are tested quarterly or annually. The score reflects the most recent data available, which may not capture very recent changes.
Emerging contaminants. Regulatory limits exist only for a defined list of parameters. Substances not yet regulated are not reflected in the score, even if they are present.
This is not medical advice. If you have health concerns related to your water, please consult a healthcare professional and contact your water supplier.