How does the WHO classify water hardness?

Where does this classification come from?

It appears in the WHO background document on hardness in drinking water, prepared for the fourth edition of the Guidelines for Drinking-water Quality and published in 2011. The document attributes the specific bands to McGowan (2000). The exact figures and the source are in the table on this page.

Is there a health guideline value for hardness?

No, and this is worth stating plainly because it is often implied otherwise. The WHO document concludes that there are insufficient data to suggest either minimum or maximum concentrations of these minerals, and so no guideline values are proposed. The bands are a descriptive classification, not a health limit.

How does this relate to the coffee brewing target?

The brewing alkalinity target of forty milligrams per litre sits below even the soft band. That is the useful comparison: water perfectly acceptable as drinking water across all four bands can still be well above what the brewing standard asks for, which is why "my water is fine" and "my water suits coffee" are different claims.

Why is hardness reported as calcium carbonate?

Hardness is caused by a mixture of polyvalent cations, so reporting each separately would be unwieldy. The convention is to express the total as the equivalent mass of calcium carbonate, which makes different waters comparable on one scale at the cost of hiding which ions are actually responsible.

Common questions

Which band is best for an espresso machine?

None of them directly. The bands describe drinking water; the brewing target of forty milligrams per litre of alkalinity sits below the soft band, so aim at that figure rather than at a band.

Do these bands tell you how fast a machine will scale?

Not directly. They describe concentration, not deposition rate, which also depends on temperature and throughput. Use the band to locate your water, then compare it against the brewing target.