Hardness is a chemical parameter of water that represents the total concentration of calcium and magnesium ions. It is called hardness because if calcium and magnesium are present in your water, making a lather or suds for washing is "hard" to do.
Another negative aspect of hard water is the capacity to produce scale, primarily caused by calcium and magnesium salts. Hardness is usually expressed in grains per gallon or parts per million (ppm) as calcium carbonate equivalent. The degree of hardness standard as developed by the Water Quality Association (WQA) is:
| Term |
Grains/gallon |
mg/L or ppm |
| Soft |
Less than 1.0 |
less than 17.1 |
| Slightly Hard |
1.0 to 3.5 |
17.1 to 60 |
| Moderately Hard |
3.5 to 7.0 |
60 to 120 |
| Hard |
7.0 to 10.5 |
120 to 180 |
| Very Hard |
10.5 and above |
180 and above |
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