What is Water Hardness? A Complete UK Guide

What is Water Hardness?

Water hardness refers to the concentration of dissolved minerals in your tap water — primarily calcium and magnesium. When rainwater percolates through underground rock formations like chalk and limestone, it dissolves these minerals and carries them into the water supply.

In the UK, water hardness varies dramatically by region. The South East of England, supplied largely by Thames Water, Affinity Water, and South East Water, has some of the hardest water in the country. Meanwhile, Scotland, Wales, and the North West of England — supplied by Scottish Water, Welsh Water, and United Utilities — tend to have much softer water.

How is Hardness Measured?

UK water companies measure hardness in milligrams per litre (mg/l) of calcium carbonate equivalent:

  • Soft: below 60 mg/l
  • Moderately hard: 60 - 120 mg/l
  • Hard: 120 - 180 mg/l
  • Very hard: above 180 mg/l

You may also see hardness quoted in Clark degrees (multiply by 14.3 to convert to mg/l) or German degrees (multiply by 17.8).

How Does Hard Water Affect Your Home?

Limescale buildup is the most visible effect. Hard water leaves white, chalky deposits in kettles, around taps, on shower screens, and inside pipes and boilers. Over time, this can reduce the efficiency of your heating system and shorten the lifespan of appliances like washing machines and dishwashers.

Soap and detergent performance is reduced in hard water. You may need to use more washing-up liquid, shampoo, or laundry detergent to get the same results. Hard water can also leave soap scum on surfaces and make clothes feel stiff after washing.

Is Hard Water Safe to Drink?

Yes. The UK Drinking Water Inspectorate (DWI) and the World Health Organisation both confirm that hard water poses no health risk. The calcium and magnesium in hard water actually contribute to your daily mineral intake, though the amounts are small compared to dietary sources.

How to Check Your Water Hardness

The easiest way is to enter your postcode on WaterGrade for an instant reading. We pull data directly from your local water company, so you'll get the actual hardness measurement for your area, not just a regional estimate.

Frequently Asked Questions

What causes hard water?

Hard water is caused by dissolved calcium and magnesium minerals. As rainwater passes through chalk, limestone, and gypsum rock formations underground, it picks up these minerals. Areas of the UK with these geological formations — particularly the South East and East of England — tend to have harder water.

Is hard water safe to drink?

Yes, hard water is completely safe to drink. The World Health Organisation and the UK Drinking Water Inspectorate confirm there are no health risks from drinking hard water. In fact, the calcium and magnesium in hard water contribute small amounts of essential dietary minerals.

How is water hardness measured?

Water hardness in the UK is measured in milligrams per litre (mg/l) of calcium carbonate. Water below 60 mg/l is considered soft, 60-120 mg/l is moderately hard, 120-180 mg/l is hard, and above 180 mg/l is very hard. Some older sources use Clark degrees (1 Clark degree = 14.3 mg/l).

How can I soften hard water?

There are several ways to deal with hard water: install a water softener (ion exchange unit) for the whole house, use a jug filter for drinking water, add washing soda to laundry, or use limescale remover for kettles and appliances. For most people, a water softener is the most effective long-term solution.

Check Your Water Quality

Enter your UK postcode for an instant water quality report graded A-F.

Check My Postcode

Check Water Quality in Other Cities

London Manchester Birmingham Leeds Glasgow Edinburgh Liverpool Sheffield Newcastle upon Tyne Cardiff Belfast Nottingham Southampton Brighton Oxford