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What is water hardness?
What is water hardness?
Hardness comes from naturally occurring calcium and magnesium mineral salts which are dissolved from the rocks through which rain water flows. Water is harder in chalk or limestone areas than those with insoluble rock such as granite.
How is hardness measured?
Hardness is expressed as the equivalent amount of calcium carbonate in parts per million (mg/l). It can also be expressed in degrees. For example, the hardness settings for dishwashers are commonly expressed in Clark’s degrees, but check with the manufacturer’s instructions as there are also other units. The following table shows the normal ranges of hardness:
| Hardness as CaCO3 mg/l | Ca mg/l | Clark Degrees | French Degrees | German Degrees | Hardness Levels |
| 0 – 50 | 0 – 20 | 0 – 3.5 | 0 – 5 | 0 – 2.8 | Soft |
| 51 – 100 | 21 – 40 | 3.6 – 7.0 | 6 – 10 | 2.9 – 5.6 | Moderately soft |
| 101 – 150 | 41 – 60 | 7.1 – 10.5 | 11 – 15 | 5.7 – 8.4 | Slightly hard |
| 151 – 200 | 61 – 80 | 10.6 – 14.0 | 16 – 20 | 8.5 – 11.2 | Moderately hard |
| 201 – 300 | 81 – 120 | 14.1 – 21 | 21 – 30 | 11.3 – 16.8 | Hard |
| Over 300 | Over 120 | Over 21 | Over 30 | Over 16.8 | Very hard |
See a map below of the waterhard ness in England and Wales.
