Water Chemistry Simulator: Hardness, Alkalinity & Limescale
Coffee is more than 98% water — and its minerals help decide how it tastes. Play around here with how total hardness and alkalinity change extraction, liveliness (acidity) and limescale risk — and see where the documented SCA/SCAE Core Zone lies: total hardness 50–175 ppm CaCO₃, alkalinity 40–75 ppm CaCO₃.
The sliders: At the top you set the total hardness (GH) — dissolved calcium and magnesium combined — from 0 to 250 ppm CaCO₃ in steps of 5. Below it, the alkalinity, also known as carbonate hardness (KH) — essentially bicarbonate — from 0 to 150 ppm CaCO₃, also in steps of 5. These are real units: ppm CaCO₃ means milligrams of calcium carbonate per litre — exactly the figure on your water analysis sheet. If you work in degrees, the conversion table further down will help.
The results: EstimateExtraction strength, liveliness, balance and limescale risk are indicators from 0 to 100, not measured values and not lab results — and not a percentage of anything. All four come from one formula: estimated, not measured. They show you the direction your coffee is moving in, not how it will taste. Here's how to read the chart next to it: to the right your water gets harder, upwards it's more strongly buffered; the dot is the water you've set, the green rectangle is the recommended zone and the small cross inside it is the target point 68/40. On a phone we only label the zone itself in the chart — the figures for it (50–175 / 40–75 ppm CaCO₃, target point 68/40) are here in the text, so the labels don't overlap.
The basis: EstimateThe green target window is the Core Zone from the SCAE Water Chart Report, adopted in the SCA Water Quality Handbook: total hardness 50–175 ppm CaCO₃, alkalinity 40–75 ppm CaCO₃. The marked target point 68/40 comes from the SCA standard “Water for Brewing Specialty Coffee”. Its 68 mg/l refers to calcium hardness, while the slider shows total hardness — we treat the two as equal because calcium makes up the larger share of hardness in most drinking water. That's a reasoned approximation, not a measurement. How strongly hardness drives extraction and alkalinity buffers acidity is assumed in the model as a plausible curve: estimated, not measured. The direction is right; the individual figure is not a lab value. The zone and the target point themselves, on the other hand, are documented — they come from the SCA documents, not from us.
What we corrected: Until recently, this tool drew a target window of 50–120 / 30–60 ppm, but internally calculated the balance towards the centre 75/42 and said “alkalinity around 40 ppm” in the text — three different numbers for the same thing. Now the graphic, the formula and the text say the same thing: zone 50–175 / 40–75 ppm CaCO₃, target point 68/40.
Water treatment and descaling directly affect your machine. This tool is a simulator: it calculates with a simplified model, gives you pointers and is no substitute for a water analysis or your manufacturer's specifications.
What always counts is your manufacturer's user manual and water specification. If you deviate from them, you put your warranty and guarantee claims at risk. If in doubt, have your water tested and stick to the values your manufacturer specifies.
We accept no liability for damage to equipment, for limescale damage or for the loss of warranty or guarantee claims arising from the application of these recommendations. All information is provided without guarantee.
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EstimateThe four indicators above are calculations from our model, not a measurement of your water. The only measured figures are the two you set yourself — hardness and alkalinity in ppm CaCO₃ from your water analysis sheet or test strips.
Hardness and alkalinity — two levers
Total hardness (dissolved calcium and magnesium) drives extraction: these cations bind to the aroma compounds in coffee and draw them out more effectively. That magnesium-rich water, of all things, extracts the most is something you read everywhere — but it isn't proven. Hendon and Colonna-Dashwood (2014) calculated this ranking using quantum chemistry, without brewing or tasting a single coffee. Bratthäll, Figueira and Nording (2024) tested it experimentally and, at around 100 ppm — typical for drinking water — found no significant difference between magnesium and calcium; a clear difference only appeared at concentrations ten times higher. For your water, that means: how much hardness you have matters — which of the two minerals provides it is secondary. Alkalinity (bicarbonate) acts as a buffer: it absorbs acids. A little of it brings balance; too much makes coffee flat and dull, too little makes it taste sharp/sour — and can even make the water corrosive. When high hardness and high alkalinity meet heat, limescale forms and clogs up the machine.
The documented target values — zone and target point are two different things
Two things get mixed up all the time. The Core Zone is the recommended range: total hardness 50–175 ppm CaCO₃ and alkalinity 40–75 ppm CaCO₃ (SCAE Water Chart Report, adopted in the SCA Water Quality Handbook). The target point is a single value inside it: the SCA standard “Water for Brewing Specialty Coffee” specifies a calcium hardness of 68 mg/l CaCO₃ (acceptable 17–85) and a total alkalinity “at or near 40 mg/l CaCO₃”. So the 40 is a target value, not the lower limit of a window — and the 68 refers to calcium alone, while the slider above shows total hardness.
The graphic, formula and text in this tool now all say the same thing: green zone 50–175 / 40–75 ppm CaCO₃, target point 68/40. Even so, there's no perfect water for everyone: light, fruity coffees like softer water with little buffer, darker ones benefit from a bit more.
Conversion guide: degrees to ppm CaCO₃
Water suppliers and test strips work in different units; this tool uses ppm CaCO₃ (equivalent to mg/l CaCO₃). Here's how to convert:
| Unit | equals | Example |
|---|---|---|
| 1 °fH (French degree) | 10 ppm CaCO₃ | 12 °fH = 120 ppm |
| 1 °dH (German degree) | 17.85 ppm CaCO₃ | 12 °dH ≈ 214 ppm |
Expressed in degrees, the Core Zone sits at a total hardness of roughly 2.8 to 9.8 °dH (or 5 to 17.5 °fH) and an alkalinity of roughly 2.2 to 4.2 °dH (or 4 to 7.5 °fH). So if you get 12 °dH from the tap, you're at around 214 ppm, well above the zone — at 5 °dH it's around 89 ppm and you're right in the zone.
Sources: Target window — SCAE Water Chart Report (Core Zone: total hardness 50–175 ppm CaCO₃ or 2.8–9.8 °dH, alkalinity 40–75 ppm CaCO₃ or 2.2–4.2 °dH), adopted in the SCA Water Quality Handbook. Target point — SCA/SCAA standard “Water for Brewing Specialty Coffee” (calcium hardness 68 mg/l CaCO₃, range 17–85; total alkalinity “at or near 40 mg/l CaCO₃”). Cations and extraction — Hendon, Colonna-Dashwood & Colonna-Dashwood (2014), “The Role of Dissolved Cations in Coffee Extraction”, J. Agric. Food Chem. — a quantum-chemical computational study with no brewing trial and no tasting; experimental cross-check: Bratthäll, Figueira & Nording (2024), “Influence of divalent cations on the extraction of organic acids in coffee determined by GC-MS and NMR”, Heliyon 10(5), e26625 — no significant difference between magnesium and calcium at concentrations typical of drinking water.
What you can do
With a water filter you lower hardness and alkalinity (less limescale). If your water is too soft or you want to build it up deliberately, mineralisation drops from Apax Lab — such as the water mineralisation drops or the box set — let you set hardness and alkalinity precisely. With the Apax Lab TDS meter you can measure where your water stands, and our page Compose your own coffee water shows how to put it together from scratch. However good your water is: regular descaling keeps your machine healthy.
Frequently asked questions
Which water is best for coffee?
There's no perfect water for everyone. A good all-rounder sits in the SCA/SCAE Core Zone: total hardness 50–175 ppm CaCO₃, alkalinity 40–75 ppm CaCO₃. The SCA target point within it is around 68 ppm calcium hardness and 40 ppm alkalinity. That magnesium-heavy water extracts more is not proven at concentrations typical of drinking water (Bratthäll et al. 2024). What is certain: softer, lightly buffered water emphasises acidity and fruit, while more buffer makes it rounder and flatter.
What's the difference between GH and KH?
GH refers to total hardness: the content of dissolved calcium and magnesium. It drives extraction. KH refers to carbonate hardness, today mostly called alkalinity: the bicarbonate content. It buffers acids. Both are given in ppm CaCO₃, and the two together, plus heat, determine the limescale risk.
Why does my machine scale up?
Limescale (calcium carbonate) precipitates when hard, alkaline water is heated. The higher the hardness and alkalinity, the faster this happens. Filtering reduces both; even so, you should descale regularly.
How do I convert °dH or °fH to ppm CaCO₃?
1 °fH (French degree) equals 10 ppm CaCO₃, 1 °dH (German degree) equals 17.85 ppm CaCO₃. So 12 °dH becomes around 214 ppm, and 5 °dH around 89 ppm. The Core Zone of 50–175 ppm total hardness corresponds to roughly 2.8–9.8 °dH.
Does a water filter help?
Yes — a filter lowers hardness and alkalinity, and with them the limescale risk. If the result is too soft (sharp/corrosive), you can deliberately add some minerals back.
You'll find all our interactive tools in one place in our Coffee Tools.
