Water is over 90% of your espresso, and it is also the single biggest factor in how long your machine needs between cleaning. The wrong water (or many tap waters!) will coat the heating system in limescale; water that is too pure makes flat-tasting coffee. This guide, based on the research in Water for Coffee by Maxwell Colonna-Dashwood and Christopher Hendon, gives you practical targets and a simple way to test your water.
The two numbers that matter
Two separate measurements determine both scale risk and taste. Both are expressed in ppm (parts per million) as CaCO₃.
General hardness (GH) is the amount of dissolved calcium and magnesium. These minerals are what extract flavor from coffee. Too little and espresso tastes thin and sour; too much and it tastes dull and heavy.
Alkalinity (KH), also called carbonate hardness or buffer, is the amount of bicarbonate. It stabilizes acidity, but it is also the ingredient that combines with calcium when water is heated to form limescale. High KH plus high calcium leads to significant scale build-up.
Targets
Measurement | Target | Acceptable range |
General hardness (GH) | 60–80 ppm | 50–100 ppm |
Alkalinity (KH) | 40 ppm | 30–50 ppm |
Chloride | as low as possible | under 10 ppm |
A few practical notes:
Scale risk rises sharply above ~50 ppm KH combined with high calcium. If your water is above 100 ppm KH, do not put it in the machine untreated.
Magnesium is your friend. It contributes to extraction like calcium but is far less prone to forming scale. Mineral recipes and remineralization products that favor magnesium over calcium are ideal for espresso machines.
Chloride is the enemy of stainless steel. Even at modest levels, chloride ions drive corrosion inside espresso machines. Keep it under 10 ppm, and never remineralize with calcium or magnesium chloride. Use sulfate and bicarbonate based mineral products instead.
Very pure water needs minerals for taste. Distilled, deionized, or straight reverse osmosis water has near-zero minerals, so it will never form scale, but it extracts poorly and makes flat, sour espresso. Remineralize it (chloride-free) to the targets above for the best results in the cup.
Avoid sodium-softened water. Ion exchange softeners swap calcium for sodium. This prevents scale but produces flat, sometimes salty-tasting espresso.
How to test your water
A simple aquarium drop test kit (such as the API GH & KH test kit, around $10–15) measures both numbers in a few minutes:
Fill the test tube with your water.
Add the KH reagent one drop at a time, swirling, until the color changes. Each drop typically equals about 17.9 ppm. So a color change on the third drop means roughly 54 ppm KH.
Repeat with the GH reagent in a fresh sample.
A TDS meter alone is not enough. It reports total dissolved solids without telling you what they are. Water at 120 ppm TDS could be perfectly balanced or a scale hazard depending on its composition. Use it as a rough sanity check.
If you want to go deeper, look up your local water report and calculate the Langelier Saturation Index (LSI), which predicts how strongly your water will deposit scale. Barista Hustle offers a free water calculator that does the math for you.
Practical options, best to acceptable
Remineralized RO or distilled water. Start from zero and add a chloride-free mineral concentrate or packet designed for coffee (e.g., Third Wave Water), following the espresso machine dosage. This gives you exact, repeatable water.
Bottled water that fits the targets. Check the label or the brand's water report for calcium, magnesium, bicarbonate, and chloride values. Many popular still waters are far too hard; low-mineral waters around 50–100 ppm total dissolved solids are usually closest.
Filtered tap water, if tested. A carbon filter (e.g., a standard pitcher filter) improves taste and removes chlorine, but it does not meaningfully reduce the dissolved calcium and magnesium that cause scale. Test the filtered output with your drop kit before trusting it.
How often should you descale?
As a rule of thumb, based on total dissolved solids:
Under ~30 ppm TDS: scale will essentially never form.
30–120 ppm TDS: descale about once a year.
Over 120 ppm TDS: descale at least twice a year, or more often depending on your water. Better yet, switch to a water source in the target range.
The harder and more alkaline your water, the faster scale builds up. If your water sits at the top of the ranges above and you pull many shots per day, descale more frequently.
What to look for
White crust or film on the surface of the chamber, or white flakes in the tank: mineral deposits are forming. Retest your water.
Cloudy white residue after water evaporates from a surface: your water is on the hard side.
Espresso suddenly tasting flat or chalky with no change in coffee or recipe: water composition may have shifted, common with seasonal changes in tap water
Learn more
If you want to go deeper on water chemistry and how it shapes taste, we recommend the book Water for Coffee by Maxwell Colonna-Dashwood and Christopher Hendon, now in its second edition. Decent Espresso also publishes an excellent, engineering-focused guide to water recommendations for their machines, which informed parts of this page. Barista Hustle also publishes a number of guides on water for coffee.