Hard water: what it is and where it is

Hard water is ordinary water carrying dissolved calcium and magnesium. It picks them up underground, from the rock it has passed through, which is why hardness is a matter of geology rather than of the water company.

Why the map looks the way it does

Water that has moved through chalk and limestone dissolves some of it and arrives hard. Water that has run off granite, slate and the older upland rocks dissolves very little and arrives soft. That single fact explains most of the pattern:

• The chalk and limestone belt running through the south and east of England tends to give the hardest water in the country
• Much of the Midlands sits somewhere in the middle
• The upland north and west, Wales, Scotland and Northern Ireland tend towards soft

It varies street by street rather than region by region, because a supply area can be fed from more than one source. The supplier publishes the hardness figure for each area it serves and that figure — not a national map — is the one to work from.

How to tell without looking anything up

What it actually costs a household

Scale is an insulator. Where it builds up on a heating element — in a kettle, a washing machine, a dishwasher, a combi boiler's heat exchanger, an immersion heater — the appliance has to work harder to move the same heat into the water. Left long enough it shortens the life of the appliance.

Alongside that sit the everyday irritations: more detergent and shampoo to get a lather, scale marks on taps and screens, and stiffer laundry. None of these is dangerous. Together they are the reason the softener market exists.

And what it does not do

Hard water is not a health risk. The minerals in it are ones people consume in food and in supplements, and UK drinking-water standards do not treat hardness as a contaminant. Anyone selling a softener on the basis that hard water is unsafe to drink is selling on the wrong argument.

Common questions

Is hard water bad for you?

No. The minerals that make water hard are calcium and magnesium, and drinking-water standards in the UK treat hard water as entirely safe. Hardness is a nuisance to appliances and to kettles, not a health problem.

How do I know if I have hard water?

Limescale is the tell. A furred kettle element, white crust around taps and shower heads, spots on glassware out of the dishwasher, and soap that will not lather easily all point the same way. The supplier publishes the hardness for each supply area and will confirm it.

Does hard water damage a boiler?

Scale builds up on heat exchangers and in hot-water cylinders, which makes them work harder for the same output. How much it matters depends on the system, the water and how hot it runs.