Water hardness

The hardness of water depends on the amount of calcium in it – the more it contains, the harder the water is. Find out more information below and use our postcode checker to see the hardness of your water.

When rain falls it contains no calcium but, as it flows over the land and filters through rocks, it dissolves minerals and becomes harder.

The amount it picks up depends upon the geology of the area and the hardness of water varies widely throughout the country.

The majority of water we supply is hard as it comes from underground chalk aquifers where the levels of calcium are very high, but the hardness varies from source to source. 

Hard water can lead to the formation of limescale in water systems and household appliances that can be avoided by using water softeners.

The definition of water hardness is based on the amount of calcium carbonate it contains per litre.

Definition of hardness

  • Soft water contains less than 100mg of calcium carbonate per litre
  • Moderately hard water contains between 100 and 200mg of calcium carbonate per litre
  • Hard water between 200 and 300mg of calcium carbonate per litre
  • Very hard water contains more than 300mg of calcium carbonate per litre.

The average hardness in your water source can be found by entering your postcode in the 'In your area' section on the right.

There is no UK or European standard set for the hardness of drinking water. 

We do not soften water for public consumption at any of our treatment works and the degree of hardness does not affect the quality of the drinking water.

While hard water does not produce as much lather, this has no effect on the performance of soaps and washing detergents.

Modern dishwashers have built-in water softeners that prevent white deposits or spots forming and water softening products can be added to older machines.


When hard water is heated, harmless limescale is formed.

Regular cleaning of kettles will prevent the 'scum' floating to the surface, while keeping the temperature of the hot water systems below 60°C will reduce the formation of limescale in boilers, on heating elements and in hot water pipes.

Find out more information on limescale. Alternatively, it can be avoided by installing a household water softener.

Water softeners must be fitted to comply with the Water Supply (Water Fittings) Regulations 1999Find out more information about Water Regulations.

If you've had a water softener installed it is essential to have one tap supplying unsoftened water for cooking and drinking as softeners can significantly increase the level of sodium in water.

Water UK: Looking after water in your home