Water
Water Hardness
The concentration of dissolved calcium and magnesium in water. Primarily a scaling and soap-performance issue rather than a health one, and separate from whether the water contains anything harmful.
In more detail
Hardness is the concentration of dissolved calcium and magnesium picked up as groundwater moves through limestone, chalk and dolomite. It varies regionally, is published for most public supplies, and is a simple test for a private well.
The effects are physical rather than toxicological. Hard water reacts with soap to form the film on shower doors and the dullness in laundry, and its dissolved minerals precipitate as scale when heated — inside water heaters, on fixtures, in coffee equipment, across the heat exchanger of a tankless unit. Scale is an efficiency and maintenance problem that compounds over years.
Two reporting units are in common use and are easy to confuse: grains per gallon, and mg/L as calcium carbonate. One grain per gallon is about 17.1 mg/L. USGS describes water above roughly 120 mg/L as hard and above 180 mg/L as very hard, thresholds that are conventions for discussion rather than health limits.
Why it matters
Hardness determines whether a house needs a softener, and that decision carries plumbing consequences — a drain, a salt supply, a bypass for outdoor spigots — that are cheap at rough-in and awkward later. It also interacts with equipment choice: tankless water heaters and steam showers are more sensitive to scale than a conventional tank, so the same water can be unremarkable in one house and a recurring service call in another.
The common misunderstanding
That hard water is unhealthy and soft water is clean. Hardness is not a contaminant and has no drinking water health limit — the EPA treats it under secondary standards, which cover aesthetic and nuisance effects. Softened water is not purified water either; two minerals have been exchanged for another, and everything else in it is unchanged.
Related terms
On Compound
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Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.