Every coffee beverage consists of at least 90% water. Filter coffee even consists of over 98% water. Less than 2% are dissolved coffee particles (TDS). These figures illustrate how important water is for coffee preparation. Coffee water should be flavorless and free from chlorine or other unwanted flavor contaminants.
But even if the water from your tap tastes good, it's not necessarily the best coffee water. The amount of minerals in your water has a decisive effect on the taste of your coffee and the longevity of your coffee or espresso machine.
In our coffee water guide, we recommend the right water hardness for filter coffee and espresso. You'll find out how to determine the hardness of your water. We explain which filtration method will bring your water into the target range. Water from some regions, even after filtration, is not suitable for brewing delicious coffee. A "mineral water calculator" helps you decode every mineral water label and find the right coffee mineral water to purchase. Another formula calculates the mixing ratios between two waters to achieve a target water.
The foundations for this article were developed by Michel Aeschbacher. Many thanks for the support to Otto Wyss from BWT and Dr. Marco Wellinger from ZHAW in Wädenswil.
More articles about coffee filtration:
- Filtering with water filter pads
- Filtering with water filter cartridges
- Filtering with table water filters
- Total hardness = Magnesium Mg+2 and Calcium Ca2+
- Alkalinity = Bicarbonate = HCO3-
- Total hardness: 2 – 3 °dH
- Alkalinity: 1 – 2 °dH
- Total hardness: 3 – 6 °dH
- Alkalinity: 2 – 4 °dH
- For calcium, it's 40mg/L versus 100mg/L bicarbonate. So the factor is 2.5.
- For magnesium, it's 24mg/L, and the factor is accordingly 4.1.
- For bicarbonate, the factor is 0.82
- That means: calcium in mg/L divided by 7.1 plus magnesium in mg/L divided by 4.35 equals total hardness.
- The bicarbonate value in mg/L is divided by 21.8 to calculate alkalinity in dH.
- Cations = Positively charged equal to or greater than total hardness (sodium, potassium)
- Anions = Negatively charged equal to or greater than alkalinity (sulfate chloride)
- Total mineralization in mg/L includes all minerals in the water as well as salts.
- Conductivity also measures all minerals in the water, expressed in micro siemens μ S/cm.
- Often all values are also given in ppm (parts per million)
- The pH value is also measured, which in most water is around the neutral value of 7. Rainwater, for example, is already acidic due to the carbon dioxide it contains, with a pH of approximately 5.5
- First, reduce total hardness to approximately 14° dH with a sodium exchange.
- Then reduce both to 4° dH GH and 2° dH alkalinity with a conventional decarbonizer.
What is good coffee water?
Good water for coffee preparation is very soft. Soft water has absorbed a low amount of dissolved and decomposed minerals.
"Soft" refers to "hardness." For coffee preparation, we're mainly interested in two measurements: total hardness and alkalinity.
At this point, a water chemist would launch into a lecture. But for now, only the essentials matter.
Total hardness and alkalinity are given in different units of measurement. We provide our recommendations for good coffee water in German degrees of hardness, but we'll explain below how you can convert all other units.
Perfect water for filter coffee:
Perfect water for espresso:
Alkalinity vs. Carbonate Hardness: On test kits for measuring water hardness and in other places, you'll see "carbonate hardness" instead of "alkalinity." We'll address this below and use the more precise term "alkalinity" in this text.
Do you need to know more to brew good coffee? Not necessarily. If you have access to this water, then enjoy your coffee. If not, read on.
What kind of water do I have at home? Let's measure together
It's easiest when perfect coffee water comes straight from your tap. In some regions, it does. A few years ago, Benjamin traveled through Switzerland tasting and measuring water from springs, and brewing coffee by the same protocol. In many regions of Valais, perfect water flows from the tap.
Water hardness can be measured with various tools. A conductivity meter can provide information just as well as a ppm meter. The simplest and cheapest way to measure is a drop test (titration test). With this, you can easily measure total hardness and alkalinity.
10 ml is placed in a clean plastic test tube. Then the test drops are added one by one. Each drop represents one degree of hardness. When you reach the hardness level of your water, the mixture in the test tube changes color. This indicates the effective hardness of your home water.
How and what does the table water filter filter?
A table water filter does its job. You can find them from well-known brands like BWT or Brita and other smaller brands. The table water filter decarbonizes only half of the water with each filtration. The other half only flows through an activated carbon filter. In 50% of the water, total hardness and alkalinity are not affected, while the other 50% of the water is decarbonized to a maximum.
Let's calculate an example. We go through another example in our water video.
From our tap, we measured water with a hardness of 16° dH total hardness and 12 °dH alkalinity. That's our starting point. From 50% of the water, we remove all alkalinity and exactly the same amount of total hardness, since we can only remove pairs of total hardness and alkalinity.
From 12 °dH, we get 6 °dH alkalinity. We also reduce total hardness by 6 °dH because it's removed as a pair. The result after the first pass is 10 °dH GH and 6° dH alkalinity.
Now we can halve the alkalinity a second time as well, but the difference of 4°dH to the GH remains. So after the second filtration, we achieve a value of 7° dH GH and 3°dH alkalinity. We're now almost in the target range for desired coffee water hardness. Another filtration gets us into the perfect target range for espresso with 5.5 °dH GH and 1.5° dH alkalinity.
Admittedly, a labor-intensive process. But one that works in many cases.
The water filter with fixed water connection
Water filters that are connected to the mains water supply have significantly higher filtration effectiveness due to line pressure. Here, larger quantities of water can be filtered in a single pass to the target range and beyond.
If the water becomes too soft, it can be adjusted to a desired target level by blending with tap water. With the filter cartridge as well, the difference between total hardness and alkalinity remains.
During decarbonization, ion exchange takes place where magnesium and calcium are replaced by hydrogen ions bound to bicarbonate. This reduces both total hardness and alkalinity proportionally. The pH value increases the more strongly we filter.
For whom is a fixed water connection the right solution?
If you drink a lot of coffee and have hard water, then a permanently installed filter system is an excellent solution. It's very convenient and guarantees the longevity of your high-quality equipment. In relation to the purchase costs of an espresso machine and grinder, the costs for installing a filter system are manageable. Over 90% of all machine failures are due to water that's too hard.
Installation on your home water line should be done by professionals, especially in rental apartments. Ultimately, however, it's not complicated. The fixed water filter can be connected to the cold water line in your kitchen via a T-fitting. If additional drilling into the countertop isn't allowed in rental apartments, we recommend simply repurposing the cold water tap as a "filter water tap." Instead of inserting a T-fitting into the line, all cold water is routed completely through the filter and drawn from the tap.
The hot water remains "normal" water and can be used without accelerating the consumption of the filter cartridge.
Mineral water as coffee water
Who hasn't experienced this? You're on vacation and want to brew delicious coffee. Your hand grinder and favorite beans are ready. But the tap water tastes like chlorine or smells unpleasant, and the coffee doesn't taste good.
Mineral water from the supermarket can be the solution. But the information on the bottle is overwhelming. What do sulfate and chloride have to do with my total hardness? How do I figure out the total hardness to find the right coffee water?
Our mineral water calculator can help you. Basically, you only need three values: calcium, magnesium, and bicarbonate.
Mineral water bottles typically list the total mineralization in mg/L. With this, we can calculate German hardness. Calcium is heavier than magnesium but lighter than bicarbonate.
Read the mg/L from your water bottle for calcium, magnesium, and bicarbonate and enter these into the formula. Below, for those interested, we provide the derivation.
Volvic example:
| Mineral | Value | Calculation |
|---|---|---|
| Calcium | 12 mg/L | ÷ 7.1 = 1.69° d |
| Magnesium | 8 mg/L | ÷ 4.35 = 1.84° d |
| Total hardness | 3.53° d GH | |
| Bicarbonate | 74 mg/L | ÷ 21.8 = 3.39 |
| Total alkalinity | 3.39° d Alk | |
The calculation is based on the amount of calcium or magnesium we need in relation to bicarbonate so that limescale forms.
When we apply these, we get a value in parts per million calcium carbonate, which tells us the effective amount of particles. Since this must then be divided by 17.85 to achieve German hardness, we take the direct route without an intermediate step.
Ignore all other values on the bottle for now.
Mixing mineral water and tap water
Mixing can do the trick too. Harder water and very soft water together can create the perfect water. A higher tap water proportion reduces the cost of buying mineral water.
The following calculation helps you mix two waters so that your target water is created. Have fun mixing.
Desalination systems are a coffee water problem!
Desalination systems supply entire houses with water. Here too, ions are exchanged, namely calcium and magnesium for sodium. This greatly reduces the total hardness of the water. However, alkalinity remains.
Alkalinity can also be described as acid buffering capacity. In nature, this is a good effect because acidic water is "reversed." For coffee, this is problematic. Coffee—like many complex beverages—thrives on acids that balance and stabilize the coffee flavor. When these acids are buffered by high alkalinity, the coffee becomes flat and dull. The finest coffees in the world taste like ordinary coffee and lose their character. That's why desalination systems are advantageous for reducing limescale formation but problematic for preparing coffee water.
In houses with desalination systems, there's usually only the option of going outside to the garden. As a rule, garden hoses aren't connected to the desalination system. Here, the process of measuring and filtering via decarbonization or switching to mineral water begins again.
Desalination systems are not suitable for preparing good coffee water!
Basics about water
Minerals are fundamentally uncharged. Dissolved in water, they break down into charged fragments called ions.
Carbonate hardness refers to the equality between alkalinity and mostly calcium, since limescale can form from these. Example: 20° dH total hardness and 12 °dH alkalinity equals 12° carbonate hardness
We measure using different methods:
| Unit | ppm CaCO₃ | °d | °f | Ca²⁺ + Mg²⁺ (mmol/L) | HCO₃⁻ (mmol/L) | Ca²⁺ (mg/L) | Mg²⁺ (mg/L) | HCO₃⁻ (mg/L) |
|---|---|---|---|---|---|---|---|---|
| 1 ppm CaCO₃ = | 1 | 0.05603 | 0.1 | 0.009991 | 0.01998 | 0.4004 | 0.2428 | 1.219 |
| 1 °dH = | 17.85 | 1 | 1.785 | 0.1783 | 0.3566 | 7.147 | 4.334 | 21.76 |
| 1 °fH = | 10.00 | 0.5603 | 1 | 0.09991 | 0.1998 | 4.004 | 2.428 | 12.19 |
| 1 mmol/L (Ca²⁺+Mg²⁺) = | 100.1 | 5.608 | 10.01 | 1 | – | 40.08 | 24.30 | – |
| 1 mmol/L (HCO₃⁻) = | 50.04 | 2.804 | 5.004 | – | 1 | – | – | 61.02 |
| 1 mg/L (Ca²⁺) = | 2.497 | 0.1399 | 0.2497 | 0.02495 | – | 1 | – | – |
| 1 mg/L (Mg²⁺) = | 4.118 | 0.2307 | 0.4118 | 0.04114 | – | – | 1 | – |
| 1 mg/L (HCO₃⁻) = | 0.8202 | 0.04595 | 0.08202 | – | 0.01639 | – | – | 1 |
Where does the difference between total hardness and alkalinity come from?
Basically, water consists of equal parts total hardness (magnesium and calcium) and alkalinity (bicarbonate). But this is often not the case because another mineral is added: gypsum. Gypsum consists of calcium (positive) and sulfate (negative). This is usually the difference between GH and alkalinity and cannot be reduced with normal filters. So the difference between GH and alkalinity remains the same even after filtering.
Only desalination would help, but it should be adjustable so that not all magnesium and calcium are exchanged.
Practical example: raw water with 20° dH GH and only 12° dH alkalinity.
Alternatively, reverse osmosis can also exchange all mineralization and re-enrich it with magnesium and calcium.
Risk zones for limescale formation and corrosion
Source: https://www.bmlrt.gv.at/wasser/wasserqualitaet/grundwasser/karte_haerte_GW2017.html
Source: Water hardness map Germany: Mensch, Wasser, Sonne, Wärme & Umwelt, Issue 1997, Environmental brochure of the Tegernseer specialist group e.V.
