If you are on a municipal water system, the water supplier can tell you the hardness level of the water they deliver. If you have a private water supply, you can have the water tested for hardness. There are several different scales used to describe the hardness of water in different contexts.
•mmol/L (millimoles per litre)
•mg/L calcium carbonate equivalent
•grains/gallon (gpg). 1 gr/US gal = 17.11 mg/L
•parts per million weight/volume (ppm w/v or ppm m/v)
Various obsolete "degrees":
English degrees (°E) Clark degrees (°Clark) - conversion to mg/L calcium: divide by 0.175. One degree Clark corresponds to one grain of calcium carbonate in one Imperial gallon of water which is equivalent to 14.28 parts calcium carbonate in 1,000,000 parts water.
American degrees - One degree American corresponds to one part calcium carbonate in 1,000,000 parts water (1 mg/L or 1 ppm)
Degrees of general hardness (dGH)
One degree of general hardness corresponds to 10 mg of calcium oxide or magnesium oxide per litre of water
The precise mixture of minerals dissolved in the water, together with the water's acidity or alkalinity (pH) and temperature will determine the behaviour of the hardness, so single number on a scale does not give a full description. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
Soft: 0 - 20 mg/L as calcium
Moderately soft: 20 - 40 mg/L as calcium
Slightly hard: 40 - 60 mg/L as calcium
Moderately hard: 60 - 80 mg/L as calcium
Hard: 80 - 120 mg/L as calcium
Very Hard >120 mg/L as calcium
As a guide, 60% of the UK and 85% of the US population live in a hard water area.
Most water softener companies will be able to supply you with a free water testing kit. Once you've tested your water supply, the hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.
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