Hard water and limescale: what UK households need to know

If you have moved house in the UK, you may have noticed that the water behaves differently from one town to the next. In some places soap lathers easily and kettles stay clean for years. In others, a white crust appears inside the kettle within weeks and a film forms on top of every cup of tea. The difference is water hardness.

What is hard water?

Hard water contains more dissolved calcium and magnesium. These minerals are picked up as rainwater passes through rock such as chalk and limestone on its way to rivers, reservoirs and underground aquifers. The more time water spends in contact with these rocks, the harder it becomes.

The Drinking Water Inspectorate (DWI) classifies water by its hardness, measured as milligrams of calcium carbonate per litre (mg/l):

• Soft: up to 100 mg/l

• Moderately soft to slightly hard: between 100 and 200 mg/l

• Hard: 200 to 300 mg/l

• Very hard: over 300 mg/l

Is hard water bad for you?

No. There is no health-based standard for hardness in UK drinking water regulations, and hard water is safe to drink. The calcium and magnesium it contains are minerals your body uses anyway. The problems with hard water are practical rather than medical.

What limescale does in your home

When hard water is heated, some of the dissolved minerals come out of the water and form limescale. According to the DWI, this scaling on heating elements makes appliances less efficient and can shorten their life. You will also notice:

• a white or grey crust in kettles, on shower heads and around taps;

• soap and shampoo that lather less, so you use more;

• “tide marks” on sinks, baths and glass shower screens;

• a scum on the surface of tea and coffee.

How to find out how hard your water is

The DWI publishes a hardness map for England and Wales, but hardness can vary within a region and even between neighbouring streets. The most accurate answer comes from your water company: most let you check your hardness online by entering your postcode, or you can ask them directly. If you do not know who supplies your water, the DWI website has a tool to find your local water company.

Home test strips are also available and give a quick indication, but your water company's figure is based on regular laboratory testing.

What helps against limescale

Filter jugs

Most filter jug cartridges contain ion exchange resin, which reduces the minerals that cause limescale in the water you drink and boil. This is the easiest option if your main concern is the kettle and your tea. In very hard water areas, the cartridge's capacity is used up faster, so changing it on time matters even more.

Plumbed-in filters

Some under-sink and tap filters also include scale-reducing media. Check the specification: not every filter reduces hardness — many are designed mainly for taste and chlorine. Our guide What does a water filter actually remove? explains the difference.

Water softeners

A water softener treats the water for the whole house using ion exchange, which protects boilers, showers, washing machines and pipes. It is the most thorough solution, but also the most expensive, and it needs ongoing salt and maintenance.

The DWI makes two important points about softeners:

• Keep an unsoftened supply to the kitchen tap for drinking and cooking, because softened water has a higher sodium content.

• Do not use artificially softened water to make up baby formula.

The DWI also notes that softeners need regular maintenance, including backwashing and topping up salt, and suggests considering a service agreement.

Simple everyday habits

• Boil only the water you need — less water heated means less scale.

• Descale the kettle regularly with a descaler or a solution of citric acid, then rinse well.

• Soak shower heads and tap aerators in a descaling solution every few months.

• Wipe down taps and glass after use to prevent marks building up.

• Use the right amount of detergent and, in dishwashers, keep the salt compartment topped up if your machine has one.

The short version

Hard water is safe to drink, but it is hard on your kettle, appliances and cleaning time. Check your hardness with your water company, use a filter jug if you mainly want less scale in drinks, and consider a softener only if you want to protect the whole house — keeping an unsoftened tap for drinking water. If your water also tastes odd, read Why does my tap water taste of chlorine?