Water is 98% of filter coffee and its mineral content changes extraction directly.
What the minerals do
Magnesium binds preferentially to the compounds responsible for fruit and sweetness. It is the most effective extractor of the flavours people want.
Calcium also extracts, less selectively, and is the main contributor to limescale.
Bicarbonate — alkalinity — buffers acidity. It neutralises the acids in coffee, which is why high-alkalinity water gives flat, chalky cups.
The targets
Total hardness — 50 to 175 ppm as CaCO₃, ideally around 70 to 100. Alkalinity — around 40 ppm, and below 70.
What goes wrong at the extremes
Distilled or reverse osmosis water alone has nothing to extract with. Coffee brewed in it is thin, flat and empty regardless of grind or dose. This surprises people who assume pure water is best.
Very hard water over-extracts harsh compounds, gives chalky cups, and destroys machines with limescale.
High alkalinity flattens acidity specifically. Bright coffee tastes dull, and no brewing change fixes it.
What to do
Filter — a jug filter reduces hardness and chlorine and is the cheap answer.
Recipe water — RO or distilled water plus a measured mineral sachet. The precise answer, and what competition brewers use.
Bottled — check the label. Look for low bicarbonate and moderate hardness.
The test
Brew the same coffee with your tap water and with a filtered or bottled water. The difference is usually larger than any grind adjustment.
Last reviewed 8 August 2026