Building water means choosing which ions to add, and they do different jobs.
Magnesium — the important one
Extracts preferentially the compounds responsible for fruit, sweetness and aromatic complexity.
It is the most effective extractor of the flavors people want, and coffee-specific water recipes usually favor it heavily.
Added as magnesium sulfate (Epsom salt) or magnesium chloride.
Calcium — secondary
Also extracts, less selectively, contributing more to body than to clarity.
It is the main contributor to limescale, so machine-focused recipes keep it low.
Added as calcium chloride or calcium sulfate.
Bicarbonate — a little, deliberately
Some alkalinity is useful. It buffers the brew, preventing the water from becoming so acidic that extraction is affected, and it prevents corrosion in machines.
Zero alkalinity water is aggressive to metal and can produce harsh, over-bright cups.
Added as sodium bicarbonate, at around 40 ppm as CaCO₃ and no more.
Sodium — avoid
Contributes nothing to extraction and adds a faint salinity. It comes along with bicarbonate of soda, which is why doses are kept small.
Potassium — occasionally
Used in some recipes as an alternative to sodium for alkalinity.
The practical route
Commercial sachets — dosed for a fixed volume of distilled water. Simple and consistent.
Home concentrates — Epsom salt and baking soda as separate stock solutions, dosed by volume. Very cheap, and it needs a 0.01 g scale.
The one rule
Never brew with the blank alone.
Last reviewed 30 October 2026