Tap Water Quality in London

Tap water in London is provided by Thames Water, which serves London & Home Counties. The water in this area is classified as very hard, with a typical hardness of around 320 mg/L CaCO₃. This means significant limescale deposits on kettles, taps, and heating elements — a water softener or filter is often a worthwhile investment for households in this area.

UK tap water is among the safest in the world, with over 99.9% of tests passing legal standards each year. However, hardness levels, trace contaminants, and the presence of disinfection byproducts like trihalomethanes (THMs) vary significantly by zone — even within the same city. That's why it's worth checking your specific postcode.

How hard is London water?

London's water hardness of approximately 320 mg/L CaCO₃ places it in the very hard category. Hard water is caused by chalk and limestone rock that water filters through before reaching the water table. In London, this is primarily from the chalk aquifer beneath the area.

For context, hardness below 60 mg/L is considered very soft, 60–120 is soft, 120–180 is slightly hard, 180–250 is moderately hard, 250–310 is hard, and above 310 mg/L is very hard. The UK average is around 150–200 mg/L.

Should I filter my water in London?

UK tap water is safe to drink straight from the tap without filtration. However, at 320 mg/L CaCO₃, London's water is very hard — a filter or water softener can significantly improve taste, reduce limescale, and extend the life of your appliances. Reverse osmosis systems remove the widest range of contaminants including nitrates, THMs, and any trace PFAS compounds.