Hard water is water with a lot of dissolved calcium and magnesium, picked up from the ground it flows through, so it depends on local geology. It is not a health risk: UK drinking water regulations set no standard for hardness (Drinking Water Inspectorate). What it does is leave limescale in kettles, boilers and showers, and make soap lather less.
Is your water hard? Check your postcode
How hardness is measured
Hardness is measured as calcium carbonate (CaCO₃) in milligrams per litre. Some appliance manuals use degrees instead: English (°Clark), German (°dH) or French (°fH). The Drinking Water Inspectorate's bands, in all four units:
| Band | mg/l CaCO₃ | °Clark | °dH | °fH |
|---|---|---|---|---|
| Soft | mg/l CaCO₃:up to 100 mg/l | °Clark:up to 7.0 °Clark | °dH:up to 5.6 °dH | °fH:up to 10.0 °fH |
| Slightly hard | mg/l CaCO₃:100–150 mg/l | °Clark:7.0–10.5 °Clark | °dH:5.6–8.4 °dH | °fH:10.0–15.0 °fH |
| Moderately hard | mg/l CaCO₃:150–200 mg/l | °Clark:10.5–14.0 °Clark | °dH:8.4–11.2 °dH | °fH:15.0–20.0 °fH |
| Hard | mg/l CaCO₃:200–300 mg/l | °Clark:14.0–21.0 °Clark | °dH:11.2–16.8 °dH | °fH:20.0–30.0 °fH |
| Very hard | mg/l CaCO₃:more than 300 mg/l | °Clark:more than 21.0 °Clark | °dH:more than 16.8 °dH | °fH:more than 30.0 °fH |
What hard water does at home
- Limescale on heating elements. Scale builds up in hot water systems, kettles, irons and appliances. It shortens the elements' life and makes them less efficient.
- Less lather. Soap, washing-up liquid and washing powder lather less, so you use more of them.
- Marks and scum. Tide marks on basins, sinks, baths and toilets, and a scum on hot drinks, especially tea brewed in the cup.
- Skin and hair. Less lather means more soap is needed to wash. A shower filter is not a softener: it does not remove hardness.
Source: Drinking Water Inspectorate, Water Hardness / Hard Water.
Is hard water bad for your health?
No. UK drinking water regulations set no standard for hardness, calcium or magnesium. The World Health Organisation's 2017 guidelines note that water of 200 mg/l or more produces scale, and that soft water of 100 mg/l or less is more corrosive to pipework.
Where water companies soften water before supplying it, the Drinking Water Inspectorate recommends they keep at least 150 mg/l of total hardness, because there is some limited evidence of a link between water hardness and heart health, which may come from the calcium and magnesium in the diet.
Softened water and drinking
Most home softeners use ion exchange: they swap the calcium and magnesium in the water for sodium. The Drinking Water Inspectorate advises keeping an unsoftened tap for drinking. Fitting a softener just before the hot water tank or boiler is a cheaper way to stop limescale than softening the whole supply.
What to do next
Frequently asked questions
- What is hard water?
- Water with a lot of dissolved calcium and magnesium. The Drinking Water Inspectorate classes water from 200 to 300 mg/l (as calcium carbonate) as hard, and more than 300 mg/l as very hard.
- Is hard water bad for your health?
- No. UK drinking water regulations set no standard for hardness, calcium or magnesium (Drinking Water Inspectorate). Hard water's effects are limescale and less lather from soap.
- Does hard water cause limescale?
- Yes. Hard water leaves scale in kettles, hot water systems, irons and other appliances. Scale on heating elements shortens their life and makes them less efficient (Drinking Water Inspectorate).
- Can I drink softened water?
- Keep an unsoftened tap for drinking. An ion-exchange softener replaces calcium and magnesium with sodium, and the Drinking Water Inspectorate advises keeping an unsoftened outlet for drinking.
- How do I find out how hard my water is?
- Enter your postcode on our water hardness page. It shows your supply zone's figure in mg/l, °Clark, °dH and °fH, with its band on the Drinking Water Inspectorate's scale.
Sources
- Drinking Water Inspectorate — Water Hardness / Hard Water ("Water Hardness (mg/l CaCO3)") ↗ · Water Hardness / Hard Water · Open Government Licence v3.0 · read 20 September 2026
Contains public sector information licensed under the Open Government Licence v3.0