Water for Coffee: Hardness, Alkalinity and TDS

Hardness, alkalinity and TDS explained

These measurements describe different properties. They are connected, but none can replace the others.

Hardness: mainly calcium and magnesium

Total hardness measures the combined contribution of dissolved calcium and magnesium. Calcium hardness measures only calcium’s contribution, so the two figures are not interchangeable.

Reports commonly express hardness in mg/L as calcium carbonate, written as CaCO₃. This is a comparison unit, not a statement that all the minerals are dissolved calcium carbonate. A calcium concentration on a bottled-water label is therefore not directly comparable with a hardness figure expressed as CaCO₃.

For dilute drinking water, ppm, meaning parts per million, is approximately equivalent to mg/L. Always keep the rest of the unit: “100 ppm hardness as CaCO₃” means something different from “100 ppm TDS”.

Alkalinity: resistance to added acid

Alkalinity is water’s capacity to neutralise acid. In ordinary drinking water, bicarbonate is usually its main contributor.

This matters because coffee supplies acids of its own. Higher alkalinity neutralises more of those acids and can make a bright coffee taste muted; lower alkalinity leaves more acidity apparent. Neither extreme is automatically desirable.

Alkalinity is not the same as pH. pH describes how acidic or alkaline the water is at the moment of measurement; alkalinity describes its acid-neutralising reserve. Two waters with similar pH can behave differently when coffee is added.

TDS: a total without an ingredient list

TDS means total dissolved solids. It describes the combined dissolved material, not which minerals are present.

A handheld water TDS meter normally estimates this figure from electrical conductivity. It cannot distinguish calcium from sodium or tell you how much bicarbonate is present. Two waters showing the same TDS can therefore produce different coffee and have different implications for equipment.

Use a meter to monitor consistency in a known supply or treatment system, not as a stand-alone certificate of good coffee water.

How minerals affect extraction and flavour

Extraction is the movement of soluble material from ground coffee into water. Minerals can interact with coffee compounds, but “harder water extracts more” is too simple a rule.

A 2014 computational study by Hendon and colleagues modelled interactions between dissolved ions and selected coffee compounds. It provided a molecular explanation for why calcium, magnesium and sodium might behave differently. It was not a brewing trial proving that a particular magnesium level makes every coffee taste better.

A 2024 laboratory study by Bratthäll and colleagues tested calcium and magnesium salts added before and after brewing. At concentrations typical of drinking water, the researchers found limited changes in the concentrations of the organic acids studied. Similar results from pre-brew and post-brew additions suggested that interactions after extraction could also matter.

These findings address different questions rather than establishing a simple winner. Mineral composition can affect the cup, but changes in flavour should not automatically be described as changes in extraction yield.

Practically, do not diagnose every sharp cup as under-extracted or every dull cup as over-extracted. Check water alongside the variables covered in grind size and extraction, while keeping the distinction explained in over-extraction and under-extraction.

What the SCA water recommendations actually mean

Figures described online as “the SCA water standard” come from different documents. The classic SCAA Water for Brewing Specialty Coffee, revised in 2009, gives the following targets.

The later 2016 SCAE Water Chart discusses a broader sensory region of roughly 50 to 175 ppm total hardness and 40 to 75 ppm alkalinity, both expressed as CaCO₃. These are not identical to the older calcium-hardness figures.

Do not combine the older TDS target with a later total-hardness range and present the result as one unchanged specification. Nor should these brewing recommendations override a machine manual.

For a beginner, the useful lesson is not to chase every boundary. Establish your water’s hardness and alkalinity, remove distracting chlorine where necessary, then compare cups within the limits appropriate to your equipment.

Check your local UK water

Regional tendencies are useful background, but your supply zone is what matters. Thames Water describes its region’s water as hard, reflecting its chalk and limestone geology. Scottish Water says most Scottish raw water comes from surface sources and tends to be soft to slightly hard, with harder supplies in some groundwater areas.

Those descriptions are not substitutes for a local analysis. Start with your supplier’s postcode search or request the report for your supply zone.

Look for:

• Total hardness and its units.
• Calcium and magnesium concentrations, if reported.
• Alkalinity, sometimes listed separately from hardness.
• Whether results describe untreated tap water or water after your own treatment.

If a report gives hardness but not alkalinity, ask the supplier rather than assuming they are equal.

Equipment instructions may use other units. One German degree, °dH, is approximately 17.85 mg/L as CaCO₃; one Clark degree is approximately 14.25 mg/L as CaCO₃. Convert before comparing your report with a filter setting or machine specification.

Filters, bottled water and recipe water

Match the filter to the task

Activated carbon and hardness treatment perform different functions. BRITA’s published technical information describes activated carbon as reducing substances such as chlorine, while ion-exchange media alter the ions responsible for hardness.

A carbon-only filter should not be assumed to soften water. Some jug cartridges combine carbon and ion exchange, but their effect depends on the cartridge and incoming supply. BRITA describes its relevant jug filters as partially demineralising water, not producing distilled water.

A conventional sodium softener replaces calcium and magnesium with sodium. That reduces hardness, but does not make its output equivalent to low-mineral water.

For a workplace machine, ask the installer to specify treatment from a water test and the machine’s requirements. Request an itemised written quotation identifying the cartridge, commissioning, replacement arrangements and maintenance responsibilities.

Read bottled-water composition

Bottled water is an alternative source, not a coffee-quality guarantee. Compare the stated calcium, magnesium and bicarbonate content rather than choosing by “spring”, “mineral” or a low TDS number alone.

Check the current bottle or producer’s analysis. If the information is insufficient to establish suitability, particularly for a boiler machine, choose a source with a fuller analysis rather than guessing.

For a manual brewer, bottled water can be useful as a comparison against tap water. You are testing whether changing the water improves your coffee, not proving that bottled water is inherently better.

Treat recipe water as measured preparation

Recipe water is made by adding measured minerals to a known base water. Published recipes commonly separate a hardness concentrate from a bicarbonate buffer, allowing those properties to be adjusted independently.

Use a complete, reputable recipe or a preparation kit’s instructions. The exact salt matters: magnesium sulphate heptahydrate, for example, includes water within its crystals, which affects the mass required. Do not substitute a different chemical form by equal weight.

Use water and ingredients explicitly suitable for consumption, with clean containers suitable for drinking water. Never reuse a container that held a non-food chemical.

Do not assume that a recipe intended for manual brewing is suitable for an espresso or bean-to-cup machine. Barista Hustle explicitly cautions against putting its remineralised recipes into espresso machines without understanding the equipment implications.

Scale versus flavour: two separate decisions

Heating hard water can produce calcium carbonate deposits. These deposits can restrict water flow and reduce heat-transfer efficiency. However, hardness alone does not fully describe scale potential: alkalinity, pH and temperature also matter.

Flavour and equipment protection therefore need separate checks. Water that tastes good in a pour-over is not automatically appropriate for repeated heating in a boiler.

Manufacturer requirements can be narrower than general brewing guidance. For example, La Marzocco’s published specification lists total hardness of 70 to 100 ppm and alkalinity of 40 to 80 ppm. These are that manufacturer’s requirements, checked in November 2026, not universal targets for other machines.

Equally, “no scale” does not establish that water is equipment-safe. Corrosion risk depends on composition and operating conditions too. Follow the relevant manual and supplier advice rather than pursuing the lowest possible mineral reading.

A practical water check and comparison

Use this suggested comparison to investigate flavour without changing several brewing variables together.

1. Establish the baseline. Obtain your local report and check any machine-specific water limits.
2. Choose a comparison water. Use a drinking-water source with a known analysis, suitable for the equipment involved.
3. Brew matched samples. For a manual filter comparison, use your established recipe, for example 15 g coffee to 250 g water, approximately 250 ml, at a coffee-to-water ratio of 1:16.7.
4. Keep the process fixed. Use the same coffee, grind, brewer, temperature and pouring method. Record the temperature and brew time rather than changing them between samples.
5. Taste as both cool. Compare acidity, sweetness, bitterness and clarity, then repeat before deciding.

For selecting the underlying recipe, see brewing ratios and temperature.

These are diagnostic leads, not certain causes. If the comparison changes little, return to bitter or sour coffee troubleshooting.