Methodology

How we gather water composition data, what we cover and what we don't, and the regulatory framework that sits behind any number on this site.

Scope

The Water Dictionary holds two kinds of data.

Bottled waters. 134 brands across 16 countries, weighted towards what is widely stocked in the UK, what appears in published recipes for brewing, coffee, baking, aquariums, and tea, and what is referenced in specialist food and drink writing. Each entry carries the six core ions (calcium, magnesium, sodium, sulphate, chloride, bicarbonate) plus pH and TDS where available, and a narrative covering origin, mineral character, documented use, and availability.

UK tap water zones. 1,569 water supply zones from 20 UK companies, covering roughly 95% of England by population. Each zone is mapped to its postcode districts, with the same six-ion profile.

For both, the underlying numbers are published with their source, and any value that is estimated rather than measured is flagged on the page.

How bottled water compositions are gathered

The primary source for each entry is the producer's own published analysis. In the UK and EU this is regulated: a natural mineral water must publish its analytical composition, either on the label or in technical documents on the producer's website (UK Food Information Regulations 2014, EU Regulation 2009/54/EC).

Where producer data is unavailable or incomplete, we use, in order of preference:

1. Retailer product pages (Sainsbury's, Tesco, Asda, Waitrose, equivalents in other markets). These typically reproduce the producer's analysis.
2. Independent geological surveys, where available. The British Geological Survey publishes profiles for several UK mineral water sources.
3. Specialist publications and named industry references: published water chemistry books, named food and drink writers, documented competition specifications.

The source and date of each measurement are recorded on every page. The bulk of the bottled water dataset was last refreshed in March 2026.

We do not measure water ourselves.

Why bottled water values vary

A bottled water is not a fixed substance. Its composition shifts for several reasons.

Natural fluctuation at source. A spring's mineral profile changes with rainfall, season, and the slow weathering of the rock it passes through. Producers publish a representative analysis, not a guarantee of any specific bottle. Variation of ±10–20% on minor ions and tighter on the majors is normal.

Multiple sources under one brand. Some brands draw from more than one source. Aqua Pura, for example, has historically drawn from springs in Cumbria and Shropshire with measurably different profiles. The bottle does not always tell you which one filled it.

Label rounding and reporting differences. Producers publish to varying levels of precision, in different units, in different countries. We normalise to mg/L and the six-ion baseline, but the underlying figures are only as accurate as what was published.

Reformulation and ownership change. Brands occasionally change source or process. Where we know, we note it. Where we don't, the figure on the page may lag the bottle on the shelf.

Practical implication: any composition on this site is a snapshot of the producer's typical analysis, not a measurement of the bottle in your hand. For most uses (coffee, brewing, baking, broad comparison) this is fine. For applications where small ion differences matter, treat the figure as a guide and verify against the label.

The legal baseline

Bottled water in the UK and EU sits in three categories, each with its own definition.

Natural mineral water is the most strictly regulated. Under SI 2007/2785 (England) and EU Regulation 2009/54/EC, it must come from a single, protected underground source, have a stable composition within natural fluctuations, be bottled at source, and not be subject to treatment beyond a narrow list of permitted processes. Each source is recognised individually by the relevant food authority and its composition is published and audited.

Spring water comes from an underground source and meets drinking water standards but is not held to the natural mineral water composition test. It may be treated to a limited degree.

Bottled drinking water is everything else: municipal supplies that have been filtered, mineralised, or otherwise processed. There is no requirement for a specific source.

When a Water Dictionary page describes a water as “Natural mineral water”, that is a regulatory designation, not a marketing one. Where a brand does not hold that status, we do not use the term. The Food Standards Agency maintains the UK register of recognised natural mineral waters; equivalent registers exist in other EU member states.

A multi-source brand is required to declare each source and publish each analysis. In practice this is sometimes incomplete, which is the gap our page on each brand tries to fill.

Measured vs estimated values

Some values are calculated rather than directly measured. They are flagged in the data and on the page.

• Hardness is calculated from calcium and magnesium using the standard CaCO₃ equivalence: 2.497 × Ca + 4.118 × Mg.
• Alkalinity is calculated from bicarbonate (0.8202 × HCO₃) where alkalinity is not separately measured.
• Bicarbonate (HCO₃) is sometimes derived from alkalinity (× 1.22) where the producer publishes only the latter.
• Calcium and magnesium, in some tap water zones, are estimated from total hardness where the company publishes hardness but not the individual ions. The estimate uses a Ca:Mg ratio drawn from regional geology.
• Sulphate, chloride, sodium, in some tap water zones, are estimated from ion balance against the measured values.

Estimated ions appear on the page with a tilde (~) prefix and are listed in the zone's underlying data file. Measured values carry no marker. Profiles with too few measurements to be useful are flagged as incomplete rather than filled in with guesses.

Tap water data

UK tap water profiles come from each water company's own published reports. The route varies.

Most companies publish annual zone-level water quality reports as PDFs on their websites. We extract values from those.

Several companies (Severn Trent, Yorkshire, United Utilities, Anglian, Southern, Welsh, Northumbrian, South West, Affinity, South Staffs, Cambridge, Hafren Dyfrdwy) publish data via the Stream Water Data portal at streamwaterdata.co.uk, which serves DWI compliance data through ArcGIS feature servers. We pull from those APIs.

Where a company does not publish zone-level data, we have used the Environmental Information Regulations to request it directly. Data received this way is noted on the relevant zone's page.

Tap water composition reflects what is being delivered to a service area, not what comes out of any specific tap. Local plumbing, building age, and stagnation in pipes can change what reaches the kitchen sink. The values on this site are a reliable guide to what the company is delivering; they are not a measurement of any individual property.

Most tap water data on the site is from compliance years 2023, 2024, or 2025, depending on each company's reporting cycle.

Corrections

If a value on this site does not match your bottle, your label, or your local supply, we want to know. Send a photo of the label or a link to the source you are working from to justin@stach.uk.

Composition data lags reality somewhere, always. The best version of this site is one where readers help keep it honest.