Water for Coffee: What to Check and How to Fix It
Brewed coffee is roughly 98% water. Whatever comes out of your tap is not a neutral carrier — it is the solvent doing the extraction, and its mineral content decides which compounds leave the coffee bed and end up in your cup. You can buy excellent beans, dial in a grinder, control your temperature, and still get a flat or harsh cup because of water you never thought to question.
This guide covers the three things that actually matter, the target numbers published by the Specialty Coffee Association, and the one method that gives you the same water every single day.
1. Chlorine: remove it first
Chlorine and chloramine are added to municipal water for a good reason — they keep it safe to drink. They are also the single most destructive thing you can put in a brew. Chlorine reacts with the aromatic compounds in coffee and produces a chemical, medicinal, swimming-pool note that sits on top of everything else. It does not extract more or less coffee; it masks the coffee you already extracted.
The SCA standard for chlorine is unambiguous: zero. There is no acceptable range. A simple activated carbon filter removes it, and so does a proper reverse osmosis stage. If you do nothing else in this guide, do this one.
2. Hardness: the minerals that pull flavour out
Calcium and magnesium are what physically bind to and carry flavour compounds out of the coffee grounds. Too little and extraction stalls: the cup reads thin, watery, hollow, no matter how fine you grind. Too much and you over-extract the harsh, drying end of the spectrum — plus you start laying scale inside your boiler, which is an expensive problem rather than a taste problem.
Total dissolved solids (TDS) is the practical proxy most people measure, because a cheap TDS meter costs very little and gives you a number in seconds. It is not a perfect measurement — it does not tell you which minerals are present — but it tells you immediately whether you are in the right neighbourhood.
3. Alkalinity: the buffer that decides how acidity reads
Alkalinity is the least understood parameter and the one that changes cups most dramatically. It is water's capacity to neutralise acid. Coffee is full of acids, and those acids are most of what we describe as brightness, sweetness and fruit.
Low alkalinity lets acidity through: the cup reads vivid, sometimes sharp or sour. High alkalinity buffers it away: the cup reads round and heavy, but also muted, dull, flat — "good coffee that tastes like nothing in particular." If you have ever brewed a celebrated washed Ethiopian and wondered where the florals went, high alkalinity is the first suspect.
The SCA water standard
The Specialty Coffee Association publishes target values for brewing water. Treat the targets as a destination and the ranges as acceptable territory — not as a pass/fail exam.
| Parameter | Target | Acceptable range |
|---|---|---|
| Total dissolved solids (TDS) | 150 mg/L | 75–250 mg/L |
| Total alkalinity (as CaCO3) | 40 mg/L | 40–70 mg/L |
| Calcium hardness (as CaCO3) | 51–68 mg/L (3–4 grains) | 50–175 mg/L |
| pH | 7.0 | 6.5–7.5 |
| Sodium | ~10 mg/L | below 30 mg/L |
| Chlorine | 0 mg/L | 0 mg/L |
| Odour and colour | Clean, clear, odourless | — |
Note: the SCA revised its water handbook, and you will find slightly different hardness and alkalinity figures quoted in older articles that cite the original 2009 SCAA table. The TDS target of 150 mg/L and the zero-chlorine requirement have not changed.
Why distilled water alone is not the answer
Once people learn that tap water is the problem, the obvious move is to eliminate minerals entirely: distilled water, or the output of a reverse osmosis system. It sounds correct and it does not work.
Water with no minerals has nothing to bind flavour compounds with. Extraction drops, and the cup comes out thin, papery and strangely empty — noticeably worse than mediocre tap water. Distilled water is also mildly aggressive toward metal: it will slowly leach from boilers and group heads, and most espresso machine manufacturers explicitly advise against running it.
Distilled or RO water is not the finished product. It is the blank canvas — and that is exactly what makes it useful.
The method: start from pure water and add minerals back
Filtering tap water is subtraction, and subtraction is imprecise. You never know quite what you removed, your utility's water changes with the season, and the result drifts.
Building water is addition, and addition is measurable. You start from something with nothing in it, then add a known quantity of minerals. Same number every time, same water every time, regardless of what the tap is doing this month.
- Start from a blank base. Distilled water, or RO water. Check the label: "purified" and "mineral" bottled waters are not blank and vary considerably by brand and batch.
- Add a measured mineral concentrate. A concentrate is a dosing instrument: a fixed volume per litre gives you a known TDS, a known calcium-to-magnesium ratio, and a known alkalinity. This is the step that replaces guesswork with a number.
- Mix, then verify once. A TDS meter confirms you are near 150 mg/L. Once you trust the dose, you stop measuring and just dose.
- Taste against your old water. Brew the same coffee, same recipe, two waters, side by side. This is the only part that matters, and the difference is not subtle.
Reading your cup backwards
Water problems have signatures. If you recognise one of these, start with water before you touch your grinder:
- Thin, watery, hollow — grinding finer doesn't help. Not enough mineral content. Your water is too soft.
- Flat and muted. Technically fine, says nothing. Alkalinity too high. Your water is buffering away the acidity you paid for.
- Harsh, drying, chalky finish. Too hard. Check your boiler for scale while you are at it.
- A chemical or medicinal note over everything. Chlorine. Filter it.
- Inconsistent from week to week, same recipe. You are drinking whatever your utility sent that week. Build your water instead.
Frequently asked
Do I need a TDS meter?
It helps once, to confirm your dose is landing where you think. After that, a measured concentrate makes the meter optional.
Can I just use bottled mineral water?
You can, and some brands land close to target by accident. The problem is that you do not control it: composition varies by source and batch, most labels do not report alkalinity, and the cost per litre adds up fast.
Does this matter for filter as well as espresso?
It matters for both, and it is more audible in filter, where there is nowhere for a muted cup to hide. For espresso, water also decides whether you are descaling every few months or not.
Where do I start if I only change one thing?
Remove chlorine. Then build from distilled with a measured concentrate.
SUNAO carries the mineral concentrates and brewing water tools referenced in this guide. If you want the short version: start from distilled water, dose a measured concentrate, and taste it against what you are brewing today.
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