This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
Hardness is the term for the calcium or magnesium carbonate
dissolved in water as Ca++, Mg++, and HCO3- (bicarbonate) ions. There
are two measures of water hardness, hardness and alkalinity. Hardness
measures the amount of positive calcium and magnesium ions; alkalinity
the negative bicarbonate ions. Both measures are usually given in
calcium carbonate, i.e. scale, equivalent units (abbreviated as
CaCO3). This means when one unit of scale precipitates out of the
water, hardness and alkalinity measured in CaCO3 units go down by one
Alkalinity and hardness levels need not be the same, since the
bicarbonates can be associated with potassium or sodium, and the
calcium or magnesium with chlorides or sulphates. Usually, alkalinity
is less than hardness, although some mineral waters and ion exchange
softened waters rich in sodium or potassium may have higher levels of
Sometimes alkalinity is called "temporary" or "carbonate"
hardness, the difference between hardness and alkalinity, "permanent
hardness", and the hardness itself, "total" or "general" hardness.
This usage is common among aquarium owners, but does not accurately
convey how scaling works.
There are no health hazards associated with water hardness, so
it is not subject to regulation. However, hard water causes scale, as
well as the scumming and reduced lathering of soaps. Very soft waters,
exposed to air or heat, become acidic and corrosive, and can harshen
the taste of vegetables, tea, or coffee. So, several countries
including the US, UK, Canada, and Germany have issued non-binding
recommended hardness ranges. These are usually aroung 80 to 100mg/l
hardness and 50 to 60 mg/l alkalinity, figures calculated to minimize
the combined cost of scaling and corrosion in municipal piping and
domestic hotwater systems. The levels required for taste, or for low
maintenance steam boiler, spa, or aquarium operation, can be quite
Water boards call waters in the recommended range "neutral,"
those below the recommended range are called "moderately soft," below
half the range, "very soft," above the range "moderately hard", and
above twice the range "very hard." However, the exact ranges referred
to by these names varies, and one should always get the exact water