Espresso is mostly water. A double shot is a small amount of coffee and a large amount of water by mass, so the water's composition affects both flavour and the machine. Yet water is often the last variable a home barista considers, and the first one blamed when nothing else works.
This guide explains the main water properties in plain terms, what they do to espresso, and how to approach the topic without guessing. It deliberately avoids brand recommendations and precise "best" numbers, because the right water depends on your machine, your coffee, and your local supply.
The properties that matter
Hardness
Hardness is mainly calcium and magnesium dissolved in water. These minerals help extract coffee compounds and can contribute to body and sweetness. Too little and the cup can taste flat or hollow; too much and the water can taste chalky and scale the machine.
Alkalinity
Alkalinity is the water's capacity to neutralise acid. It acts as a buffer. Low alkalinity lets coffee's acidity come through sharply; high alkalinity can mute brightness and make the cup taste flat or chalky. Alkalinity is different from hardness, though the two are often discussed together.
Total dissolved solids (TDS)
TDS is a broad measure of everything dissolved. It tells you how mineralised the water is but not which minerals. Two waters with the same TDS can behave very differently.
pH
pH describes acidity or alkalinity on a scale. It matters, but for most home setups it is secondary to hardness and alkalinity. Do not chase pH first.
Chlorine and chloramine
Municipal water may contain chlorine or chloramine for disinfection. These can produce medicinal or swimming-pool notes in coffee. They also vary by supplier. A suitable filter can remove them, but the correct filter depends on which compound is present.
What water does to the shot
- Very soft water: can taste flat, thin, or papery. Extraction may be less efficient.
- Very hard water: can taste chalky and cause scale. Scale reduces flow and heat transfer over time.
- High alkalinity: can flatten acidity and dull the cup.
- Low alkalinity with high acidity coffee: can make the shot taste sharp, even when the recipe is reasonable.
- Chlorine: adds an off-flavour that no grind adjustment can fix.
This means water can mimic other problems. A flat cup might look like old coffee; a sharp cup might look like under-extraction. Before you spend a bag chasing grind, check the water.
Test your own water
Do not assume. Water varies by city, by building, and sometimes by season.
- Start with a report. Your water supplier publishes an analysis. Read hardness, alkalinity, and the disinfectant used. Reports can be broad averages, so treat them as a starting point.
- Use a simple test. A TDS meter gives a rough number, not a full picture. Hardness and alkalinity test kits give more specific information.
- Taste the water plain. Room temperature, in a clean glass. If it tastes metallic, chalky, or of chlorine, that is a clue.
- Compare. Brew with your tap water and with a filtered or bottled alternative, same coffee and recipe. If the cup changes, water is a variable.
Record the water source in your shot log when you change it. Otherwise a water change will look like a grinder problem.
Filtration and treatment
There are several approaches, and they are not interchangeable:
- Carbon filters target chlorine, chloramine, and some odours. They do not necessarily soften water.
- Softening reduces hardness but may increase sodium, depending on the method.
- Reverse osmosis removes most dissolved solids, producing very soft water that may need remineralisation for good espresso.
- Bottled water varies widely; check its composition rather than assuming "spring" means suitable.
Follow the manufacturer's guidance for your machine. Some machines are sensitive to water chemistry, and the warranty may specify water requirements. Do not assume a filter marketed for drinking water is ideal for espresso equipment.
Scale and your machine
Hard water deposits scale in boilers, valves, and passages. The practical consequences are reduced flow, noisy pumps, temperature instability, and eventually damage. Descale on a schedule that reflects your measured hardness and the manufacturer's instructions, not a generic calendar.
If you use very soft or reverse-osmosis water, be aware that some machines and some coffees perform differently. Soft water is not automatically better; it is a different trade-off.
A practical approach for most homes
- Find out what is in your water, at least roughly.
- Remove chlorine/chloramine if it is present and you taste it.
- Address excessive hardness if scale is a problem.
- Keep the source consistent, so your dialing is not chasing water changes.
- Record the source and any filter changes in your log.
Consistency matters as much as the exact composition. A stable, decent water lets you dial in; constantly changing water makes every other variable unreliable.
What not to do
- Do not add minerals or "water recipes" without understanding what you are adding.
- Do not assume distilled water is ideal; it may be too soft and can be aggressive to some equipment.
- Do not judge water by TDS alone.
- Do not descale more often than necessary; descaling is not a routine flavour fix.
Bottom line
Water affects extraction, flavour, and machine health. Learn your hardness, alkalinity, and disinfectant, remove chlorine if needed, control scale, and keep the source stable. Test before you treat, and measure your own supply rather than copying someone else's numbers.
Keep the record
Log the next shot while it is fresh.
Espresso Journal keeps the recipe, gear, and taste note together.