Gera Bathing Water Score (GBWS) — Methodology
Published 2026-06-20 · Data: Environment Agency Bathing Water Quality API (seasons 2019, 2021–2025)
What is the GBWS?
The Gera Bathing Water Score (GBWS) is a proprietary, reproducible composite index that answers: “How good and consistent is the water quality at [bathing water site or area], based on official EA data?”
It combines two real EA rBWD annual compliance classification components into a single 0–100 score where higher values indicate better and more consistent water quality. GBWS scores range from 15 (lowest) to 100 (highest) across 422 English bathing water sites with sufficient data. The national average GBWS across 68 local authorities is 87/100.
Sites with fewer than 3 assessed seasons are shown as “insufficient data” — the score is never invented or estimated.
Data source
- Source
- Environment Agency — Bathing Water Quality API
- Publisher
- Environment Agency
- API endpoint
- https://environment.data.gov.uk/doc/bathing-water.json
- Compliance endpoint
- https://environment.data.gov.uk/data/bathing-water-quality/compliance-rBWD/point/{id}/year/{year}.json
- Seasons covered
- 2019, 2021–2025 (2020 excluded: COVID-19 cancelled the bathing season)
- Fetched
- 2026-06-20
- Licence
- Open Government Licence v3.0
Formula
// Step 1: Map the 2025 classification to a score
latest_score = { Excellent: 100, Good: 75, Sufficient: 50, Poor: 25 }[class_2025]
// Step 2: Compute pass rate over all assessed seasons (up to 6: 2019, 2021-2025)
pass_rate = n_pass / n_seasons
where n_pass = count of seasons rated Excellent/Good/Sufficient
and n_seasons = total seasons with a published assessment
// Step 3: Weighted composite
GBWS_raw = 0.6 × latest_score + 0.4 × (pass_rate × 100)
// Step 4: Round and cap
GBWS = round(GBWS_raw) // 0–100
// Step 5: Insufficient data guard
if n_seasons < 3: GBWS = null (shown as "insufficient data")
- latest_score
- Numerical score for the 2025 EA rBWD classification: Excellent=100, Good=75, Sufficient=50, Poor=25. This is the current water quality snapshot. Weight: 0.6.
- pass_rate
- Proportion of all assessed seasons available for the site (up to six: 2019, 2021–2025, excl. 2020) where it was rated Sufficient or above (i.e. passed the minimum rBWD standard). A site rated Excellent in all six seasons has pass_rate=1.0; one with four Poor and two passing seasons has pass_rate≈0.33. Weight: 0.4.
- LA GBWS
- The mean GBWS of all sites within the local authority that have sufficient data (n_seasons ≥ 3). Sites with insufficient data are excluded from the LA average but listed on the LA page.
Score distribution — sites (422)
| Band | Sites | % of total |
|---|---|---|
| Outstanding (90–100) | 293 | 69.4% |
| High (75–89) | 90 | 21.3% |
| Good (60–74) | 19 | 4.5% |
| Moderate (0–59) | 20 | 4.7% |
27 sites have fewer than 3 assessed seasons and are excluded.
Score distribution — local authorities (68)
| Band | LAs | % of total |
|---|---|---|
| Outstanding (90–100) | 42 | 61.8% |
| High (75–89) | 17 | 25.0% |
| Good (60–74) | 6 | 8.8% |
| Moderate (0–59) | 3 | 4.4% |
National average GBWS: 87/100 across 68 local authorities.
Limitations and disclaimers
- Annual snapshot, not real-time. The GBWS is based on the 2025 annual classification published by the EA. It does not reflect day-to-day conditions. After heavy rainfall or pollution incidents, water quality may be temporarily worse than the annual rating. Always check for EA advisories before swimming.
- rBWD regime only (2015+). Only assessments under the Revised Bathing Water Directive regime are used. Older cBWD assessments (pre-2015) use a different classification system and are not included. This means some long-established sites may have fewer seasons of rBWD data than their total monitoring history suggests.
- 2020 excluded. The 2020 bathing season was cancelled due to COVID-19. No assessments were published for that year and it is correctly excluded from all calculations.
- Insufficient data sites. 27 of 449 designated sites have fewer than 3 assessed seasons and show “insufficient data”. These are typically newly-designated sites or those that had monitoring gaps. Their latest 2025 classification is shown where available.
- LA GBWS is a mean, not a legal rating. The LA-level GBWS averages individual site scores. A high LA GBWS means most sites in the area are good; it does not mean every site is safe on every day.
- Not a substitute for official advice. The EA’s real-time advisories and in-season sample results (available at environment.data.gov.uk/bwq/) should always be consulted before swimming.
Explore bathing water quality by area
See how 75 local authorities compare on the Gera Bathing Water Score.
View UK Bathing Water Quality rankings →Contains public sector information licensed under the Open Government Licence v3.0. Source: Environment Agency — Bathing Water Quality API (https://environment.data.gov.uk/doc/bathing-water.json). Fetched: 2026-06-20. Annual compliance classifications under the Revised Bathing Water Directive — not real-time water quality data.