Hard water explained: what it means for your home
Hard water carries a lot of dissolved calcium and magnesium. It is safe to drink, but it leaves limescale in kettles, showers, boilers and washing machines.
What makes water hard
Rain falls soft. As it soaks through chalk and limestone, it picks up calcium and magnesium. The total of those two minerals is the water's hardness. Much of the south and east of England sits on chalk, which is why towns there tend to have hard water, while the north and west, on granite and peat, usually have soft water.
Calcium and magnesium are useful minerals, so hard water is not a health risk. Whether tap water is safe depends on germs and pollutants, which water companies test for constantly, not on its hardness.
How hardness is measured
English water companies usually give hardness in milligrams of calcium carbonate per litre (mg/L CaCO3). Some still quote Clark degrees, an old British unit: one Clark degree is 14.25 mg/L. You may also see mmol/l or German degrees (°dH) on appliance manuals. This site uses mg/L throughout and shows Clark degrees alongside.
The bands used on this site:
- soft: under 100 mg/L (under 7 °Clark)
- moderately hard: 100 to 199 mg/L (7 to 14 °Clark)
- hard: 200 to 299 mg/L (14 to 21 °Clark)
- very hard: 300 mg/L or more (21 °Clark and above)
Temporary and permanent hardness
Part of the hardness is bicarbonate, which drops out as white scale when water is heated. That is temporary hardness, and it is what coats your kettle and the element in your immersion heater. The rest, permanent hardness, stays dissolved, and it is what leaves marks on glasses and shower screens as water dries.
Signs of hard water at home
White crust in the kettle, chalky rings around taps, a film on tea, spotty glasses after the dishwasher and shower heads that spray sideways are all typical. The guide to measuring hardness shows how to get the exact figure for your address.
What to do next
Measure first, then decide. Most homes only need the dishwasher set correctly, the right detergent dose and regular descaling. A water softener is worth pricing only when scale is clearly damaging a boiler, a combi or appliances. The guide comparing softeners, filters and descalers explains how to weigh it up.