13 September 2026 · 3 min read
a cup of coffee is 98% water, so its make-up shapes flavour as much as the beans and the grind. and it isn't about «clean vs dirty» — it's about the minerals dissolved in the water: they are what extract and hold flavour.
the two key numbers
two figures govern coffee water. general hardness (gh) is calcium and magnesium; they grab the aromatic compounds from the bean, so hardness drives body and fullness. alkalinity (kh, carbonates) is a buffer: it tames acids. the higher the kh, the quieter and rounder the cup; the lower it is, the brighter and more sour.
the targets
the industry reference (from sca material and the book «water for coffee») is a general hardness of about 50–175 mg/l as caco3, with a comfortable target around 60–70, and alkalinity near 40. it's not dogma but a window in which specialty coffee sounds balanced.
what's wrong with tap water
it behaves differently city to city: chlorine adds a medicinal note, excess carbonates «mute» acidity and flatten the cup, and scale builds up on your gear. a simple carbon filter removes chlorine and some impurities — a noticeable win for almost nothing — but it doesn't set the minerals you want.
a recipe for coffee water
the most controllable way is to start from «zero» (distilled or ro) and add the minerals yourself. the basic kit: epsom salt (magnesium sulfate) for hardness without scale, and baking soda (sodium bicarbonate) for the alkalinity buffer. make two concentrates separately (a teaspoon of each salt per litre of distilled water), then dose a little of each into a large jug of distilled water until you hit the numbers. magnesium instead of calcium is deliberate: a cleaner taste, and no limescale on the boiler.
espresso machines — carefully
in a machine the water is also heated, so scale is the main enemy. keep hardness lower and alkalinity moderate (here alkalinity also protects the boiler from corrosion). never run pure distilled water with no minerals through a machine: it's aggressive to metal and some sensors won't «see» it.
when it's worth it
if your tap water already tastes soft and pleasant, start with a filter jug — often that's enough. recipe water is the next level: once your beans and grind are dialled in and you want to squeeze the most out of the cup, with repeatability. the difference between «whatever» water and water built for coffee is audible even to a beginner when you taste them side by side.