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Water hardness

Enter postcode for water hardness information

Our water supply is generally hard, having percolated slowly through the overlying layers of limestone, chalk or similar rock, dissolving calcium and magnesium salts on the way.

The rest comes from surface water stored in reservoirs. The hardness of this run-off will also depend upon the type of soil and rock present in the surrounding grounds.


Measuring hardness

The hardness of water is expressed as milligrams per litre of calcium carbonate (mg CaCO3/l). This is the equivalent of parts per million (ppm).

As a general indication water is classified as follows:

  • hard water – more than 200 mg CaCO3/l
  • moderately hard water – 100-200 mg CaCO3/l
  • soft water – less than 100 mg CaCO3/l.

There is no maximum limit for the hardness of drinking water.

Our region

Our region lies across five main rock formations, three of which produce naturally hard water.

Location Rock formation Hardness
Cotswolds Oolitic limestone 250-350 mg CaCO3/1
West Wiltshire Greensand 200-250 mg CaCO3/1
Salisbury Plain and Dorset Downs Chalk 230 - 280 mg CaCO3/1
Exmoor and Quantock Hills Devonian sandstone silt stone and slate 50 - 150 mg CaCO3/1
Blackdown Hills Greensand and chalk 50-150 mg CaCO3/1

Common indications of hard water

  • Poor lathering of soap.
  • Limescale
    • White or coloured particles in hot water from kettles.
    • Floating white scum on boiled water.
    • Deposits or scale on taps and basins.
    • White film covering all or part of items on glassware or items from dishwashers.

Find out more about limescale and how to reduce it.

Dishwasher settings

Some dishwashers require setting to a water hardness level to get the best performance. With hard water, the inbuilt water softener uses more salt to work effectively. The inbuilt softener can also prolong the life of your dishwasher. Please refer to your appliance manual for instructions on how to set the hardness level.

Find out the hardness of the water in your area. The results are given in several different units. Please ensure that you use the value with the units corresponding to those asked for in the appliance instructions.

Softening water

We don’tartificially soften water as part of the treatment process. Nor do we recommend softened water for drinking and cooking purposes.

Water softening is a personal choice as some people prefer hard water. Some would like softer water; others prefer it to be naturally hard. Customers can make their own decision about softening water and installing a domestic water softener in their property.

To find out more about water softening, please see our page on softeners and conditioners.


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