Water for Coffee Guide: TDS, Hardness, Filtration, Why It Changes Everything

Updated: 5 September 2026

3 key takeaways

• Coffee is 98 to 99% water, so the mineral content of that water is not a side setting. It is the substrate of the drink.
• Three numbers describe brewing water: TDS, calcium hardness and alkalinity. The SCA puts them at 150, 68 and 40 mg/L, the last two expressed as CaCO₃.
• Rebuilding water from reverse osmosis takes two food-grade salts, a scale accurate to 0.01 g and about five minutes per litre.

Coffee is 98 to 99% water. That single fact should be enough to settle any debate about whether water quality matters. Yet the vast majority of home coffee enthusiasts who invest in a premium grinder and single-origin specialty beans continue to brew with unfiltered tap water, the one variable most likely to be silently sabotaging every cup. This guide breaks down what your water is actually doing to your coffee, why TDS and hardness are the two numbers worth understanding, and which filtration solutions are worth considering at every budget level.

What does a TDS reading actually tell you about brew water?

TDS, total dissolved solids, is the mass of minerals, salts and other substances dissolved in a litre of water, quoted in mg/L or in parts per million, two units that are interchangeable for water. At zero, you have theoretically pure water: no flavour of its own, but no extraction capacity either. Above 250 mg/L, the water starts imposing its own character and competing with the aromatics you are trying to pull out.

The SCA target sits at 150 mg/L. Below 75 mg/L, whether from reverse osmosis or a very soft spring source, extraction runs thin: the most soluble compounds come out too fast and the cup lands unbalanced, sometimes astringent. Here is the catch a pocket meter will never tell you. Those meters infer TDS from electrical conductivity and cannot distinguish a bicarbonate from a sulphate. Two waters both reading 150 mg/L can behave nothing alike in the brewer.

How do temporary and permanent hardness differ in a coffee machine?

Hardness and TDS get confused constantly, yet they measure different things. Hardness counts only calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Temporary hardness, also called carbonate hardness, comes from calcium and magnesium bicarbonates: that is the fraction which precipitates as limescale when water is heated. Permanent hardness comes from sulphates and chlorides and stays in solution whatever the temperature.

For a coffee machine, temporary hardness is the practical enemy. It coats heating elements, group heads and boilers, degrades thermal stability and eventually causes expensive failures. Standard residential softeners swap calcium and magnesium for sodium on an ion-exchange resin. The scale risk goes away, but so does the useful mineral content, and what comes out is soft, flat water that extracts poorly.

One note on units, because this is where most comparisons fall apart. One French degree (°f) is 10 mg/L of CaCO₃; one German degree (°dH) is roughly 17.8 mg/L. Water quoted at 30 °f is about 300 mg/L CaCO₃, which is 16.9 °dH, not 30 °dH.

Does magnesium really extract coffee aroma better than calcium?

Yes, on the chemistry. Christopher Hendon, Lesley Colonna-Dashwood and Maxwell Colonna-Dashwood calculated the binding energies of dissolved cations to coffee acids in the Journal of Agricultural and Food Chemistry in 2014, and magnesium came out ahead of calcium and well ahead of sodium. In the cup, water carrying 15 to 25 mg/L of magnesium pulls more floral and fruity detail, especially from light-roasted washed Ethiopians. Calcium extracts too, but less selectively, and it lands on body rather than on aromatic definition.

Which is why two waters reading an identical 150 mg/L TDS are not interchangeable. A magnesium-sulphate water and a calcium-bicarbonate water are different extraction media, because the bicarbonate buffers the acids while the sulphate lets them through. That is the whole reason competition baristas write recipes rather than chase a single TDS number.

Mineral, filtered or reverse osmosis: how do the three compare?

* depending on setting and source water. Volvic and Montcalm figures come from the published label analyses; the Brussels row uses the hardness ranges published by the utility.

Brita, Peak Water, BWT or reverse osmosis: what does each remove?

Brita pitcher filters. The most accessible option. Standard Brita cartridges reduce chlorine, some heavy metals, and a portion of limescale through ion-exchange resin. Resulting TDS varies heavily with source water: in a hard-water city like Brussels (above 300 ppm tap), the output may still be 200+ ppm. Adequate for batch brew, limiting for espresso. Advantage: widely available, low entry cost.

Peak Water Pitcher. Designed specifically for coffee, this filter lets you adjust filtration level via a slider, directly setting the output TDS from your source water. It is the most targeted domestic solution for the serious home brewer who wants water control without committing to reverse osmosis. Available in Belgium at around €60-80.

BWT Bestmax and Bestprotect. Professional-grade filters used in specialty coffee shops. They combine mechanical filtration, chlorine adsorption, and controlled calcium/magnesium exchange with magnesium remineralization. Excellent for fixed espresso machines, but sized for commercial throughput; the BWT Penguin range is the accessible domestic equivalent.

Reverse osmosis with remineralisation. The complete solution for full water control. An RO system removes 95-99% of dissolved minerals; you then remineralize to your target composition with calibrated food-grade salts. Setup cost is €150-300, but marginal cost per liter is nearly zero thereafter. Recommended if you own an espresso machine worth €1,000 or more.

How do you mix a lab water recipe for espresso?

Start from reverse osmosis or distilled water at almost zero TDS, then add back only the two salts that earn their place: magnesium sulphate for hardness and extraction, sodium bicarbonate for buffering. Both dosages below are calculated against the SCA targets rather than copied from a forum.

Balanced recipe, on the SCA target. Dissolve 170 mg of magnesium sulphate heptahydrate (MgSO₄·7H₂O, food-grade Epsom salt) and 67 mg of sodium bicarbonate per litre of RO or distilled water. That gives roughly 17 mg/L of magnesium, so about 69 mg/L CaCO₃ of hardness, alkalinity close to 40 mg/L CaCO₃ and a TDS near 150 mg/L. No calcium is added, and that is deliberate. Two caveats travel with this recipe: the hardness you get is magnesium hardness, while the SCA figure is a calcium hardness, and the bicarbonate carries roughly 18 mg/L of sodium, above the 10 mg/L the standard asks for.

Fruity and floral recipe, magnesium forward. 200 mg of magnesium sulphate heptahydrate and 50 mg of sodium bicarbonate per litre. Hardness climbs toward 80 mg/L CaCO₃ while alkalinity drops to around 30 mg/L CaCO₃, so acidity reads more clearly. Good on light-roasted Ethiopians and Kenyans.

Both salts are sold in pharmacies and homebrew shops. You need a scale accurate to 0.01 g and dropper bottles if you prefer to work from stock solutions. Worth knowing: Epsom salt sheds its water of crystallisation once dissolved, so only the magnesium and the sulphate count toward TDS, which is why 170 mg of salt produces far less than 170 mg/L of dissolved solids.

Upgrading your water filter before buying a more expensive coffee is often the single best return on investment available in specialty coffee. The same recipe can produce a forgettable cup or a remarkable one depending entirely on what the water brings to the extraction.

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How do you build brewing water from mineral concentrates?

Making brewing water from distilled or reverse osmosis base water used to be a laboratory habit. It is now routine in serious home setups and in a growing number of specialty cafés, largely because the working method is simple once you decide what you are actually dosing.

Three parameters carry almost all of the sensory difference. Magnesium favours the extraction of aromatic compounds. Calcium adds mineral body but starts inhibiting extraction at high concentration. Bicarbonate, which is what alkalinity measures, buffers acidity, and too much of it flattens a bright coffee. Almost every published recipe therefore leads with magnesium, keeps calcium modest and holds bicarbonate on a short leash.

Working from stock solutions rather than weighing milligrams each time is the practical move. Dissolve a known mass of magnesium sulphate heptahydrate in a litre of distilled water, do the same separately with sodium bicarbonate, then dose the two concentrates into your brewing water by volume. The precision requirement shifts from a milligram scale to a graduated cylinder, which is far easier to get right in a kitchen. Keep the two concentrates separate: mixed and stored together, they will eventually drop magnesium carbonate out of solution.

Verification is where most people over-invest. A decent TDS meter confirms that the total landed where you intended, within a tolerance that matters little in the cup. Hardness and alkalinity test strips, the kind sold for aquariums and homebrewing, give you the split between the two numbers that actually drive flavour. Laboratory analysis through a water testing service gives a full ionic breakdown, and it is worth doing once to validate a recipe, not as ongoing quality control.

Which water treatment fits your local supply?

The right answer depends entirely on what comes out of your tap, and over-engineering is as common a mistake as ignoring water altogether.

Where the supply is naturally soft, parts of Scandinavia, much of Scotland, and the Ardenne catchments in Belgium, tap water often already sits close to the specialty target band with low hardness and moderate alkalinity. Filter brewing may need nothing at all. The only real question is whether the treatment plant leaves chlorine or chloramine at a level you can taste. An activated carbon stage, whether a pitcher or an under-sink block, deals with that without touching a mineral profile that was already fine.

Hard supplies are the harder case. In Brussels, the utility publishes hardness that ranges from roughly 14 °f on some reservoirs to over 40 °f on others, with most of the network between 25 and 35 °f, so a single citywide number is meaningless and you should look up your own address. High calcium and high bicarbonate together create scale risk in the equipment and flatten the acidity of the coffee. The professional answer is two stages: ion exchange or reverse osmosis to bring hardness down, then selective remineralisation. Cartridge systems such as BRITA PURITY C combine ion exchange with activated carbon and partial mineral retention, which is why they turn up in so many hard-water city cafés.

Reverse osmosis strips nearly everything and lands in the region of 5 to 20 mg/L TDS. It is the most thorough answer to very hard water and the one that most obviously fails if you stop there: brewing with demineralised water gives thin, hollow coffee, because the ions that carry extraction are simply absent. Pair it with a remineralisation stage, either a mineral block cartridge or the dosing approach described above. Combined units exist and make sense for cafés and restaurants that cannot run a recipe by hand at volume.