BrewWater
What water should you use to brew coffee?
Short answer
Brew with water near 150 mg/L total dissolved solids and 40 mg/L alkalinity, containing no chlorine. Filtered tap water suits most people. Distilled and reverse osmosis water taste flat and corrode machines unless you add minerals back. Water above 120 mg/L hardness scales boilers and mutes acidity.
Five water profiles compared
Hardness as mg/L calcium carbonate equivalent · ordered by increasing mineral content
Distilled / RO
0
WHO calls demineralised water highly aggressive. Tastes flat and corrodes untreated metal. A starting point, not a finished product.
Soft
under 60
Minimal scaling and efficient heat transfer. Needs enough alkalinity to stop it turning corrosive.
Moderately hard
60 to 120
The band most municipal supplies land in. Workable, with scale accumulating slowly in a heated boiler.
Hard
120 to 180
Carbonates drop out onto the hottest surface. Expect temperature stability to drift before anything visibly fails.
Very hard
over 180
Scales fastest and mutes acidity in the cup. Treat the water rather than descaling on a schedule.
| Measure | Value | Unit | Source |
|---|---|---|---|
| Target total dissolved solids for brewing water | 150 | mg/L | Specialty Coffee Association of America · verified |
| Acceptable total dissolved solids range for brewing water | 75–250 | mg/L | Specialty Coffee Association of America · verified |
| Target total alkalinity for brewing water | 40 | mg/L as CaCO3 | Specialty Coffee Association of America · verified |
| Total chlorine permitted in brewing water | 0 | mg/L | Specialty Coffee Association of America · verified |
| Upper bound of moderately hard water; above this, water is classified as hard | 120 | mg/L as CaCO3 | World Health Organization · verified |
What does good brewing water actually contain?
Two numbers carry most of the weight: dissolved mineral content, and alkalinity. Minerals give extraction something to work with, and alkalinity buffers the acids that coffee releases. Too little of either and the cup reads thin and sour; too much and it turns flat and chalky. Chlorine is the one component with no acceptable amount at all, because it survives brewing as an obvious taste.
Is filtered tap water good enough?
For most people, yes. A carbon filter removes chlorine, which is the single biggest taste problem in tap water, and it does so cheaply. What a carbon filter does not do is change hardness or alkalinity, so if your supply is hard it stays hard after filtering. That matters for your machine rather than for the cup, and it is the reason hardness and taste have to be considered separately.
Why is distilled water a bad idea on its own?
This is the claim most often got wrong. Water with the minerals stripped out is not neutral, it is aggressive. The WHO hardness background document cited on this page 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. It also notes that demineralised water tastes flat. Distilled or reverse osmosis water is a starting point to add minerals to, not a finished product.
How does hard water damage a machine?
Heating hard water precipitates insoluble carbonates onto whatever surface is hottest. The same WHO document describes scale deposition in heated water applications, where insoluble metal carbonates coat surfaces and reduce the efficiency of heat exchangers. In an espresso machine that means the boiler and the heating element, which is why a machine on hard water loses temperature stability long before it visibly fails.
Common questions
- Does bottled water work for espresso?
- Some bottled waters sit close to the brewing targets and some are far outside them. The mineral content is printed on the label, so it is a matter of reading the figures rather than trusting the brand.
- Will a water softener solve the problem?
- An ion-exchange softener swaps calcium and magnesium for sodium. That protects the boiler, but it raises sodium, which the brewing standard wants held near 10 mg/L. It solves the machine problem and creates a taste one.