Coffee is about 98 per cent water, and water does two jobs: it extracts flavour, and it either protects or destroys your machine.
The window you want
Roughly 50 to 150 ppm of total hardness — moderately soft to medium. That is enough mineral content to extract properly and little enough to keep scale slow.
Above about 150 ppm, limescale becomes a genuine threat to any machine with a boiler. Below about 50 ppm you have the opposite problem.
Why very soft water is bad
This surprises people. Extraction is not just hot water dissolving things — magnesium and calcium ions actively bind to flavour compounds and carry them into solution. Water with almost no minerals extracts poorly, and the cup tastes thin and hollow regardless of the beans.
That is why distilled or reverse-osmosis water is a mistake on its own.
What to actually use
Filtered tap water is right for most people. A jug filter or an in-tank cartridge reduces the carbonate hardness that causes scale while leaving useful minerals behind.
Bottled water can work, and you have to read the label — many mineral waters are harder than tap. Look for low bicarbonate and moderate total mineral content.
Mineral recipes — sachets dosed into distilled water — give the most control and the best cup, and they are a genuine commitment.
Find out what you have first
Your water utility publishes hardness by postcode, usually in °dH, °fH or ppm. That takes two minutes and tells you which problem you actually have. Buying a filter without knowing is guessing.
Questions people actually ask
Does chlorine matter?
Yes, and it is easy to remove. Chlorine and chloramine give a distinctly flat, swimming-pool edge that carbon filtering removes almost entirely. Any jug filter handles it.
Is the water in my kettle fine for pour-over?
If it is filtered, yes. Pour-over has no boiler to scale, so the only question is taste — and unfiltered hard or chlorinated water is noticeably worse in the cup.
Last reviewed 8 August 2026