“Hard” is the colloquial term for water that is high in calcium, magnesium and other dissolved minerals.

You know you have hard water if water leaves white rings behind when it evaporates (“limescale”).  The presence of these minerals is not a health hazard per se, but hard water can be a significant nuisance.  It can plaque up water pipes and heating coils in hot water tanks as well as clogging delicate equipment that processes hot water.  The dissolved mineral content also reduces the effectiveness of soaps by preventing good lathering, leaves white spots or haze on the dishes and windows, and can put a dull build-up on hair.


Degree of Hardness Grains per Gallon (gpg) ppm (or mg/L)
 Soft <1.0 <17.0
Slightly Hard 1.0-3.5 17.1-60
Moderately Hard 3.5-7.0 60-120
Hard 7.0-10.5 120-180
Very Hard >10.5 >180

*Water treatment companies often express hardness in grains per gallon (gpg). 1 gpg equals 17.1 mg/L.  The nomenclature of “soft” vs. “medium” hardness, etc. is a subjective rating, and the values associated with each description vary by source.

Accretion of mineral deposits from hard water in a copper pipe.

Exposure Guidelines

Not applicable; it’s a nuisance factor.  Calcium and magnesium are considered nutrients in other contexts.


“Hardness in Water,” USGS Water Science school (webpage)

Wikipedia, “Hard Water”

“Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Hardness” – Another point of view (webpage)

Wikipedia, “Water Softening”