What is hard water?
Hard water is water that has picked up dissolved calcium and magnesium on its way to your tap. That is the whole of it. Rain falls soft, lands on rock, and where that rock is chalk or limestone it dissolves a little of it. By the time the water reaches your kitchen it is carrying those minerals in solution.
Whether your water is hard is therefore a question about geology, not about your water company's diligence. Southern and eastern England sit on chalk and limestone and have hard water. Scotland, Wales, the North West and the South West peninsula sit on older, harder rock that dissolves far less, and have soft water. The line between them runs roughly diagonally across the country, and it is why a household in Kent and a household in Cumbria have almost nothing in common on this subject.
How hardness is measured
The UK standard is milligrams per litre of calcium carbonate, written mg/l CaCO₃. Parts per million (ppm) is the same number under an older name.
You will also meet degrees Clark, degrees German (°dH) and degrees French (°fH), which are the same measurement in different units, and this is where confusion creeps in. Worse, water companies often publish calcium (Ca) rather than calcium carbonate — and the calcium figure is roughly 40% of the calcium carbonate one. Read 110 mg/l Ca as though it were CaCO₃ and you will conclude your water is soft when it is in fact very hard at around 275.
| Classification | mg/l CaCO₃ | °Clark | °dH |
|---|---|---|---|
| Soft | 0–100 | 0–7 | 0–5.6 |
| Moderately soft | 100–150 | 7–10.5 | 5.6–8.4 |
| Slightly hard | 150–200 | 10.5–14 | 8.4–11.2 |
| Moderately hard | 200–250 | 14–17.5 | 11.2–14 |
| Hard | 250–300 | 17.5–21 | 14–16.9 |
| Very hard | 300+ | 21+ | 16.9+ |
What hard water actually does
The visible effects are the familiar ones: limescale in the kettle, white marks on taps and shower screens, soap that will not lather, laundry that feels stiff. Annoying, but cosmetic.
The expensive effect is invisible. When hard water is heated, the dissolved calcium comes out of solution and deposits as scale on whatever surface is doing the heating — the heat exchanger in a combi boiler, the element in a washing machine, the internal pipework of a shower. Scale is an excellent insulator, so the appliance has to work harder and run hotter to deliver the same heat, and it wears out sooner. In a very hard water area this is a real and recurring cost.
It is also why boiler manufacturers commonly make some form of scale protection a warranty condition in hard water areas. Not out of caution — because they know what they will otherwise be asked to repair.
Is hard water bad for you?
No. Drinking hard water is not harmful, and the minerals in it — calcium and magnesium — are ones the body needs anyway. There is no health reason to soften your drinking water, and the Drinking Water Inspectorate's standards are concerned with contaminants, not hardness.
Some people find hard water aggravates dry skin or eczema, and there is reasonable evidence of an association, though the mechanism is likely to be as much about needing more soap and detergent as about the minerals themselves. If that is your reason for considering a softener, it is a legitimate one — just be clear that it is a comfort question rather than a safety one.
Softened water is a slightly different matter. Ion-exchange softening replaces calcium with sodium, and in a very hard water area that adds a meaningful amount of sodium. Standard UK practice is to leave the kitchen cold tap unsoftened for drinking and cooking, and any competent installer will do this without being asked.
What to do about it
That depends entirely on how hard your water actually is, which is why every recommendation on this site starts with your postcode rather than with a product.
Broadly: below about 150 mg/l CaCO₃ there is nothing to fix. Between roughly 150 and 250, a scale inhibitor protecting the boiler is usually proportionate. Above 250 — which covers London, the South East, East Anglia and much of the Midlands — a water softener starts to make real financial sense, and most households who fit one say they notice the difference within days.
Note what none of that involves: a drinking water filter. Under-sink carbon filters do very little to hardness. If your problem is limescale, a filter is not the answer, however it is marketed.