Water Chemistry Simulator: Hardness, Alkalinity & Limescale
Your water has a say in how your coffee tastes
Filter coffee is more than 98 % water, espresso around 90 % — as a guide, espresso contains roughly 8 to 12 % dissolved solids. Whether your coffee tastes lively or flat, and whether your machine is scaled up a year from now, comes down to two numbers from your tap: the hardness and the buffer. Set both here — or tap «Zurich» or «Bern» — and see straight away where your water sits and what that means.
The sliders: «Hardness» is the dissolved calcium and magnesium (total hardness) from 0 to 300 ppm CaCO₃, «Buffer» is the bicarbonate (alkalinity) from 0 to 250 ppm CaCO₃ — with the conversion to °fH and °dH shown alongside.
The results: Model valueFour indicators from 0 to 100 out of our model — not measured values, not lab results. They show the direction, not how your coffee will taste.
The basis: The green zone and the target point come from the SCA documents, the tap-water examples from the water utilities; how strongly hardness and buffer act is assumed in the model. No guarantee — details right below and underneath the tool.
In more detail: sliders, model and sources
Units: ppm CaCO₃ means milligrams of calcium carbonate per litre — the number on your water analysis sheet. Swiss water utilities mostly work in French degrees of hardness: 1 °fH = 10 ppm CaCO₃. German degrees: 1 °dH = 17.85 ppm CaCO₃. Both sliders move in steps of 1 ppm so you can set your value exactly. We widened the slider ranges in October 2026 (previously 0–250 and 0–150 ppm), because Bern tap water at around 170 to 200 ppm alkalinity would otherwise not have fitted on the chart.
The chart: Both axes are linear and carry quantity and unit; below the hardness axis there is an extra °fH line. The dot is the water you have set, the green rectangle is the SCA zone (total hardness 50–175, alkalinity 40–75 ppm CaCO₃), the cross inside it is the target point 68/40. The orange area at the top right shows where our model calculates a high scale risk, the blue band at the bottom where there is too little buffer. Zone and target point are documented; the coloured areas come from the model. The dot always stays blue — what your water is doing right now is told by the chips and the text next to it.
The model: Model valueHow strongly hardness drives extraction and alkalinity buffers acidity is assumed as a plausible curve: estimated, not measured. We calculate the scale risk from the product of hardness and alkalinity — the two together plus heat make limescale. Balance measures the distance to the target point. «Acidity noticeable» and «Balanced» have a floor of 5 out of 100 — the model knows no «zero». The direction is right; the individual number is not a lab value.
The target values: The green zone is the Core Zone from the SCAE Water Chart Report, adopted in the SCA Water Quality Handbook (2018). The target point 68/40 comes from the SCA standard «Water for Brewing Specialty Coffee»: calcium hardness 68 mg/l CaCO₃ (acceptable 17–85), total alkalinity «at or near 40 mg/l CaCO₃». Its 68 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. A reasoned approximation, not a measurement.
New since 2024: In October 2024 the SCA published the standard SCA 102-2024 (Coffee Value Assessment: Sample Preparation and Tasting Mechanics). Its Table 2 specifies for cupping water: calcium hardness 50–175 ppm CaCO₃, alkalinity «at or near 40–70 ppm CaCO₃», pH 6–8, no chlorine. That almost matches the green zone — only the upper buffer limit is 70 instead of 75. We keep drawing the zone from the handbook and mention the new value alongside. As far as our research shows, there is no new Water Quality Handbook (as of 16.10.2026).
Tap-water examples: Zurich — Wasserversorgung Zürich, «Die Qualität des Zürcher Trinkwassers 2025», averages: total hardness 15.8 °fH (13.8–19.1 depending on the plant), carbonate hardness 14.8 °fH (12.6–18.2). Bern — Wasserverbund Region Bern AG / ewb, «Trinkwasseranalysen Mittelwerte 2020–2024»: total hardness 21 °fH, alkalinity 17 °fH (Aare valley source) or 20 °fH (Emmental source); the «Bern» button sets the Aare valley values. Both accessed 16.10.2026. Which supply your house is connected to, your water utility can tell you; the values vary over the year. «Very soft» is an example value, not a measurement.
What we corrected: Until September 2026 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. Since then, chart, formula and text say the same: zone 50–175 / 40–75 ppm CaCO₃, target point 68/40. In October 2026 we also rescaled the scale indicator: before, the top corner of the green zone (175/75) counted as «scale danger» — a contradiction within the same chart. Now that corner sits at 50 out of 100 («some scale»), Bern tap water at 100.
Water treatment and descaling interfere with your machine. This tool is a simulator: it works with a simplified model, gives pointers and replaces neither a water analysis nor your manufacturer's specifications.
What always applies is your manufacturer's user manual and water specification. Anyone who deviates from them risks their warranty and guarantee claims. When in doubt, have your water measured and stick to the values your manufacturer prescribes.
We accept no liability for damage to equipment, for scale damage or for the loss of warranty or guarantee claims that result from applying these recommendations. All information is provided without guarantee.
The water chemistry simulator is loading … If nothing appears, please reload the page — or compose your own coffee water.
Model valueThe four indicators above are calculated values from our model, not a measurement of your water. The only measured numbers are the two you set yourself — hardness and alkalinity from your water analysis sheet, from your water utility or from a test strip. Whatever you derive from them for filters, mineralisation or descaling has to fit your manufacturer's user manual and water specification.
Hardness and buffer — in everyday language
Hardness means: calcium and magnesium are dissolved in the water (technical term: total hardness, GH). These minerals are the «hands» the water uses to grab flavour compounds from the ground coffee. More hardness, more gets dissolved — the coffee seems stronger. Very soft water grabs little: thin. That magnesium-rich water of all things extracts most strongly is something you read everywhere — it is not proven. Hendon and Colonna-Dashwood (2014) calculated this ranking with quantum chemistry, without brewing or tasting a single coffee. Bratthäll, Figueira and Nording (2024) checked it experimentally and found no significant difference between magnesium and calcium at drinking-water-typical levels of around 100 ppm; it only became clear at concentrations ten times higher. For your water that means: how much hardness you have counts — which of the two minerals provides it is secondary.
Buffer means: bicarbonate is dissolved in the water (technical terms: alkalinity, carbonate hardness, KH, buffer capacity). It catches acidity. Coffee naturally brings fruity acids — a little buffer makes them round and pleasant, too much buffer swallows them entirely: the coffee tastes flat and dull. Too little buffer lets all the acids through: sharp, sour — and very soft water can attack metal parts in the machine.
Limescale forms when the two come together and get heated: calcium and bicarbonate precipitate as calcium carbonate and settle in the boiler, on valves and heating elements. The more hardness and the more buffer, the faster. That is why hardness and alkalinity always belong together — and why your manufacturer's specification stands above any calculated value.
The documented target values — zone and target point are two different things
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 within it: the SCA standard «Water for Brewing Specialty Coffee» names 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 edge of a window. New since October 2024: the SCA standard 102-2024 specifies for cupping water a calcium hardness of 50–175 and alkalinity around 40–70 ppm CaCO₃ — practically the same zone. Still, there is no perfect water for everyone: light, fruity coffees like softer water with little buffer, dark ones benefit from a bit more. If you like it, you're right.
Where Swiss tap water sits
Both cities we call home have tap water that sits above the zone — especially on buffer. That is no fault of the drinking water, it is simply Alpine limestone in the glass. For espresso it means: tending from round to flat in the cup, and a real scale issue in the machine.
| City | Total hardness | Alkalinity (carbonate hardness) | Source |
|---|---|---|---|
| Zurich averages 2025 | 15.8 °fH = 158 ppm CaCO₃ (13.8–19.1 °fH depending on the plant) | 14.8 °fH = 148 ppm CaCO₃ (12.6–18.2 °fH) | Wasserversorgung Zürich, «Die Qualität des Zürcher Trinkwassers 2025», accessed 16.10.2026 |
| Bern averages 2020–2024 | 21 °fH = 210 ppm CaCO₃ (Aare valley and Emmental) | 17 °fH = 170 ppm (Aare valley) 20 °fH = 200 ppm (Emmental) | Wasserverbund Region Bern AG / ewb, «Trinkwasseranalysen Mittelwerte 2020–2024», accessed 16.10.2026 |
The values apply at the plant outlet and vary by neighbourhood, season and mixing in the network. Which water reaches you, your water utility can tell you — or you measure it yourself.
Conversion help: degrees to ppm CaCO₃
Waterworks and test strips use different units, this tool uses ppm CaCO₃ (the same as mg/l CaCO₃). This is how you get there:
| Unit | equals | Example |
|---|---|---|
| 1 °fH (French hardness, common in Switzerland) | 10 ppm CaCO₃ | 15.8 °fH = 158 ppm |
| 1 °dH (German hardness) | 17.85 ppm CaCO₃ = 1.785 °fH | 12 °dH ≈ 214 ppm ≈ 21.4 °fH |
Expressed in degrees, the Core Zone sits at a total hardness of 5 to 17.5 °fH (around 2.8 to 9.8 °dH) and an alkalinity of 4 to 7.5 °fH (around 2.2 to 4.2 °dH). So if you get 21 °fH from the tap, at 210 ppm you are well above the zone — at 9 °fH it is 90 ppm and you are right in the middle.
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 (2018). Target point — SCA/SCAA standard «Water for Brewing Specialty Coffee» (rev. 2009; calcium hardness 68 mg/l CaCO₃, range 17–85; total alkalinity «at or near 40 mg/l CaCO₃»). New 2024 — SCA Standard 102-2024, Coffee Value Assessment: Sample Preparation and Tasting Mechanics, Table 2 «Specifications of Water for Cupping» (calcium hardness 50–175 ppm CaCO₃, alkalinity at or near 40–70 ppm CaCO₃, pH 6–8), accessed 16.10.2026. 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 counter-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 drinking-water-typical concentrations.
What you can do
First, the one rule that stands above everything: what your manufacturer prescribes in the user manual and the water specification applies — for hardness, for filters and for descaling. Within those specifications you have three levers. With a water filter you lower hardness and buffer at the same time (less scale, a livelier cup). If your water is very soft or filtered and you want to build it up deliberately, the mineralisation drops from Apax Lab help — for example the Box Set 3×20 ml to get started or the Box Set 100 ml — which let you set hardness and buffer. A good fit if you start from very soft or filtered water. With the Apax Lab TDS meter you see how much is dissolved in total, and how to build coffee water from scratch is shown on our page Compose your own coffee water. No matter how good your water is: regular descaling as specified by the manufacturer keeps the machine healthy — how to do it is in our help on water and descaling. And if you would rather sort it out in a conversation: Jackson, our AI advisor, helps you make sense of your manufacturer's specification.
Frequently asked questions
Which water is best for coffee?
There is no perfect water for everyone. A good starting point lies in the SCA/SCAE Core Zone: total hardness 50–175 ppm CaCO₃ (5–17.5 °fH), alkalinity 40–75 ppm CaCO₃ (4–7.5 °fH). The SCA target point within it is 68 ppm calcium hardness and 40 ppm alkalinity; the new SCA standard 102-2024 specifies 50–175 and 40–70 for cupping. That magnesium-heavy water extracts more is not proven at drinking-water-typical concentrations (Bratthäll et al. 2024). What is certain: softer, lightly buffered water emphasises acidity and fruit, more buffer makes it rounder and flatter.
What is the difference between hardness and alkalinity (GH and KH)?
Hardness (GH, total hardness) is the dissolved calcium and magnesium — the minerals that pull flavour out of the coffee. Alkalinity (KH, carbonate hardness, buffer) is the bicarbonate — it catches acidity. Both are given in ppm CaCO₃ or °fH, and the two together plus heat determine the scale risk.
How hard is the tap water in Zurich and Bern?
Zurich: on average 15.8 °fH total hardness and 14.8 °fH carbonate hardness (Wasserversorgung Zürich, 2025). Bern: 21 °fH total hardness and 17 to 20 °fH alkalinity depending on the source (Wasserverbund Region Bern, averages 2020–2024). Both sit above the SCA zone, especially on buffer. Which values reach you, your water utility can tell you.
Why does my machine scale up?
Limescale (calcium carbonate) precipitates when hard, buffered water is heated. The higher hardness and alkalinity, the faster. Filtering reduces both; you still have to descale — at the intervals your manufacturer specifies.
How do I convert °fH or °dH to ppm CaCO₃?
1 °fH (French hardness) equals 10 ppm CaCO₃, 1 °dH (German hardness) equals 17.85 ppm CaCO₃. 15.8 °fH becomes 158 ppm, 12 °dH around 214 ppm. The Core Zone of 50–175 ppm total hardness corresponds to 5–17.5 °fH or around 2.8–9.8 °dH.
Does a water filter help?
Yes — a filter lowers hardness and alkalinity and with them the scale risk. If the result is too soft (sharp, corrosive), you can deliberately add some minerals back. Whether and which filter suits your machine is in the manufacturer's water specification.
You will find all our interactive tools together in our Coffee Tools.
