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.