BrewWater
Distilled, reverse osmosis or filtered water for coffee — which should you use?
Short answer
Carbon-filtered tap water is the practical choice: it removes chlorine and leaves minerals intact. Distilled and reverse osmosis water strip almost everything, and the World Health Organization describes demineralised water as highly aggressive to metal. Use them only as a base you remineralise yourself.
| Option | Mineral content | Chlorine removed | Scale risk | Corrosion risk | Needs dosing | Verdict |
|---|---|---|---|---|---|---|
| Distilled | Effectively zero | Yes | None | High | Yes | A blank starting point. Aggressive to metal and flat-tasting until you add minerals back. |
| Reverse osmosis | Near zero | Yes | None | High | Yes | Removes virtually all minerals, so it behaves like distilled water and needs the same treatment. |
| Carbon-filtered tap | Unchanged from supply | Yes | Same as supply | Low | No | The pragmatic default. Fixes the taste problem without touching hardness, for better or worse. |
| Ion-exchange softened | Calcium swapped for sodium | No | Low | Variable | No | Protects the boiler but raises sodium well past the brewing target, so the cup suffers. |
| Measure | Value | Unit | Source |
|---|---|---|---|
| Total chlorine permitted in brewing water | 0 | mg/L | Specialty Coffee Association of America · verified |
| Target sodium concentration for brewing water | 10 | mg/L | Specialty Coffee Association of America · verified |
| Target total dissolved solids for brewing water | 150 | mg/L | Specialty Coffee Association of America · verified |
Why is filtered tap water usually the right answer?
Chlorine is the one component the brewing standard permits none of, and it is the single biggest taste fault in most tap water. A carbon filter removes it cheaply. What a carbon filter does not do is change hardness or alkalinity, so if your supply is hard it stays hard — that is a machine problem to solve separately, not a reason to reject the approach.
Why not just use distilled water and avoid scale entirely?
Because it trades a slow, visible, fixable problem for a faster and less visible one. The World Health Organization states that demineralised waters are highly aggressive and must be stabilised before distribution, and that unstabilised soft water has a great tendency to corrode metal surfaces and pipes. Scale you can descale; corrosion you cannot undo.
When is reverse osmosis actually worth it?
When you want repeatability. Starting from near-zero mineral content and dosing deliberately gives you the same water every time regardless of what your supply does seasonally. That is the real argument for it, and it only holds if you actually do the dosing rather than running it neat.