pH measures how acidic or alkaline water is at a moment. Alkalinity measures its capacity to resist change, and that is the meaningful number.
Why pH alone tells you little
Coffee is far more acidic than water. When you brew, the coffee’s acids dominate the pH of the result almost entirely.
Water at pH 7.5 and water at pH 6.5 will produce cups of very similar pH, if their alkalinity is the same.
What alkalinity does
Bicarbonate neutralises the coffee’s acids as they are extracted.
Water with high alkalinity and a moderate pH will flatten a bright coffee completely. Water with low alkalinity and the same pH will not.
That is why alkalinity is the number to test.
The targets
pH — 6 to 8. Almost all drinking water is in this range already.
Alkalinity — around 40 ppm as CaCO₃, below 70. This is where the variation and the problem lie.
Where pH does matter
Very low pH water — below 6 — can be corrosive to machine components over time.
Very high pH usually accompanies high alkalinity, and the alkalinity is the reason the coffee suffers.
Alkaline “health” waters
Marketed at pH 9 or above, with high alkalinity to match. Poor for coffee — they will neutralise the acidity comprehensively.
The practical summary
Ignore pH. Test alkalinity with an aquarium KH kit, and aim low.
Last reviewed 8 August 2026