Coffee is 98% water. The minerals dissolved in that water determine how flavour compounds are extracted from the grounds, which flavours are emphasised, and how the final cup tastes.
What minerals do in extraction
Calcium and magnesium are the extraction drivers. They bond with flavour compounds in the coffee grounds and pull them into solution. Water with very low mineral content (like distilled) under-extracts, producing weak, sour, flavourless coffee. Water with very high mineral content over-extracts, producing harsh, bitter, astringent cups.
Bicarbonate (alkalinity) acts as an acid buffer. Coffee is naturally acidic, and bicarbonate neutralises some of that acidity. A little bicarbonate (40–75 mg/L) rounds out brightness; too much (above 100 mg/L) flattens the cup and mutes fruit notes.
Sodium and chloride in small amounts add body and sweetness. In large amounts they produce flat, salty flavours.
Sulfate adds a dry, clean finish but can emphasise bitterness if too high.
The SCA and Rao guidelines
The Specialty Coffee Association recommends a target range for brewing water:
- Total hardness: 50–175 mg/L as CaCO₃
- Alkalinity: 40 mg/L as CaCO₃
- pH: 7.0
- TDS: 75–250 mg/L
Scott Rao's research narrows this further, advocating for lower hardness (around 50–90 mg/L) and minimal alkalinity to preserve acidity and clarity.
Both agree on the principle: moderate minerals, low alkalinity. This gives enough extraction power without buffering away the bright, complex flavours that good coffee offers.
Why city waters taste different
Melbourne has very soft water — low minerals across the board. This is part of why Melbourne's coffee culture emphasises light roasts and bright, fruity flavours. The water doesn't add heaviness.
London has hard, alkaline water from chalk aquifers. The high bicarbonate buffers acidity, which is why London cafés have historically favoured darker roasts — the water naturally suppresses the brightness of lighter roasts.
Tokyo sits in a sweet spot: moderate minerals, low alkalinity. This may explain the Japanese affinity for precision pour-over methods — the water lets subtle flavours come through clearly.
Getting started
If you're using unfiltered tap water for coffee, the single biggest improvement is switching to a controlled water. Pick one of our recommended profiles — Melbourne for bright, clean cups; Rao's ideal for balanced extraction — and try it side by side with your tap water using the same beans. The difference is usually immediately obvious.