Water hardness, alkalinity and TDS: coffee explained

Hardness is how much calcium and magnesium is dissolved in the water. French utilities call it TH, titre hydrotimétrique, and calculate it as TH = 10 × ([Ca²⁺] + [Mg²⁺]) with concentrations in mmol/L (Wikipédia, Dureté de l'eau). Baristas say GH, for general hardness. That abbreviation is borrowed from fishkeeping, honestly, because aquarium shops were selling cheap accurate titration kits long before anyone made one for coffee.

Those ions aren't passive contaminants. Magnesium drives extraction, calcium is the one that ends up as scale inside the boiler, and so the same water decides both how the cup tastes and how often you'll be descaling the machine.

Chemist Christopher Hendon, co-author of a 2014 study, put it this way: "hard water is generally considered to be bad for coffee, but we found it was the type of hardness that mattered, while high bicarbonate levels are bad, high magnesium ion levels increase the extraction of coffee into water and improve the taste."

Read that twice. It splits apart two things that nearly every article on this topic treats as one.

Not the same as TDS. Hardness counts two ions. TDS counts everything else too.

Alkalinity of water measures how well it buffers an acid, and that buffering comes from bicarbonate and carbonate. French utilities call it TAC, titre alcalimétrique complet. Baristas say KH, from the German Karbonathärte, another loan word out of the aquarium hobby. The SCA range is 40 to 70 ppm CaCO₃.

More bicarbonate means the coffee's own acidity gets buffered away, and the cup reads flat, one of the signs that let you diagnose an espresso that's too flat or too sour straight from the taste. It's probably the worst understood of the four numbers in the standard, mostly because people fold it into pH, which is only the visible consequence.

This isn't hardness by another name. Water can be soft in GH and still heavily buffered in KH: neither figure tells you the other.

TDS stands for total dissolved solids, and it adds up everything in the water: calcium, magnesium, sodium, chlorides, nitrates. The SCA range is 75 to 250 ppm, with a target of 150 ppm. Useful as an aggregate, but it says nothing about the GH to KH ratio, and two waters sharing a TDS figure can taste nothing alike.

And the big one: this is not the TDS of your cup. Brewed coffee TDS, also called strength, gets measured with a refractometer and expressed as a percentage of the drink's mass, roughly 1.15 to 1.35% in North America and 1.20 to 1.45% in Europe, alongside an extraction yield of 18 to 22% (detailed definitions). One abbreviation, two scales, two different moments. The logic traces back to Earl Lockhart's work at the Coffee Brewing Institute, published in 1957 (history of the brewing chart). None of this is new.

One French degree equals 10 mg/L CaCO₃, and the German degree that filter brands print equals 17.85 mg/L (conversion table).

Right-hand column: our own arithmetic from the factors in the middle column, not a figure the SCA publishes.

Tap water in Paris runs somewhere around 25 to 30 °f, so 250 to 300 ppm, well above the band. Brittany and the Massif Central sit on granite and come in at 5-15 °f, landing inside it with no treatment at all (regional hardness in France). So you may not have a water problem. You might just have a units problem.

Reverse osmosis water often sits below 10 to 50 mg/L of dissolved minerals (common misconceptions about RO water), which puts it under the SCA floor of 75 ppm. And coffee brewed with it doesn't come out weak, exactly. It comes out hollow and harsh at the same time.

This isn't a taste preference. Researchers at the University of Bath showed, in a 2014 paper in the Journal of Agricultural and Food Chemistry, that dissolved sodium, magnesium and calcium coordinate to nucleophilic motifs in coffee and pull them into solution. Take the minerals out and nothing is left to do that job.

Here's the irony. RO water gets sold, fairly, as the safest thing to drink. For extraction that same purity is precisely the flaw.

Three ways to find your starting number. One of them is free:

• A hardness test strip gives you one number in seconds, nothing on alkalinity or TDS.

• For the aggregate in ppm, though never the GH to KH ratio, there's a TDS meter.

• Free, and probably already in a drawer: your water bill's report.

A hardness test strip

A strip dipped for a few seconds gives you hardness in French degrees, which is enough to place yourself in the table above. It tells you nothing about alkalinity or TDS. If you only want one number, this is the one.

COMAP S900671 – languette de test de dureté (TH) de l'eau

A TDS meter

An electronic meter shows TDS in ppm within seconds. Worth knowing what it actually does: it reads conductivity and converts, so what you get back is the aggregate, never the GH to KH ratio.

HM Digital TDS-EZ – testeur TDS numérique 0-9990 ppm

The report attached to your water bill

Free, and probably already in a drawer. In France, official water quality results including hardness are published commune by commune through the Service-Public.fr portal; most utilities elsewhere publish an equivalent annual analysis.

A bottle never prints the word "hardness". It prints dry residue at 180 °C, which is the closest thing to TDS. Then calcium and magnesium in mg/L, which give you hardness through the French formula above. Then bicarbonate (HCO₃⁻), which carries the alkalinity. Four lines, and any bottled water can be placed against the SCA table.

You can read hardness, alkalinity and TDS now, and you know why those three numbers decide how your coffee tastes. What's left is the real question: which water to buy, which filter to fit, and whether a softener actually earns its keep. That one lives in what water for an espresso machine.