London water is very hard (240–330 mg/L CaCO₃). Supplied by Thames Water, Affinity Waterand others. Enter your postcode for your exact zone report.
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London is served primarily by Thames Water, one of the UK's largest water companies, supplying approximately 15 million customers across London and the Thames Valley. Smaller parts of Greater London are served by Affinity Water and SES Water. The water reaching London taps comes from two main sources: surface water abstracted from the River Thames and River Lee (stored in large ring-main reservoirs in west London), and groundwater from the chalk aquifer beneath the city.
London has some of the hardest tap water in the UK — and one of the hardest in Europe. Water hardness typically ranges from 250 to 330 mg/L CaCO₃ across different supply zones, with most central London zones around 310–320 mg/L. This extreme hardness is caused by the chalk and limestone geology that underlies London and the Thames Valley: as rainwater percolates through chalk, it dissolves calcium carbonate, which enters the water supply and is responsible for the limescale that plagues every kettle, boiler, and showerhead in the capital.
Thames Water holds an active DWI PFAS improvement notice — one of 14 UK water companies currently under regulatory enforcement. PFAS (per- and polyfluoroalkyl substances) have been detected in Thames Water supply zones. The DWI's position is that water remains safe to drink under current guidelines, but the improvement programme requires Thames Water to reduce PFAS concentrations over time. Affinity Water and SES Water have also been subject to PFAS monitoring. For households with infants, pregnant women, or immunocompromised individuals, a certified reverse osmosis (RO) filter (NSF/ANSI 58) provides maximum protection.
Unlike most UK cities where water comes primarily from moorland reservoirs, London's water comes from rivers and chalk aquifers — sources that naturally carry high mineral content. The resulting water has distinct characteristics: very high calcium (responsible for limescale), relatively high sulphate, and moderate chlorine for disinfection of the river-sourced water. The good news: London water routinely passes 99.9%+ of regulatory tests. It is absolutely safe to drink, but the hardness makes a compelling case for filtration for taste and appliance protection.
Water quality varies by supply zone rather than borough. Thames Water operates hundreds of separate supply zones across Greater London. Central and Inner London zones are typically supplied from Thames-sourced water. Outer London zones — particularly in the north and east — may draw from the Lee Valley reservoir system. Zones in Hertfordshire and north London served by Affinity Water can have slightly different hardness profiles. Enter your postcode above to see your specific zone's data.
Yes. Tap water in London is safe to drink. UK tap water is subject to rigorous regulation and testing. Over 99.9% of samples pass legal standards each year. Enter your postcode above for your specific zone's data.
London has very hard (240–330 mg/L CaCO₃) water. Heavy limescale on appliances is expected. A water softener or RO filter is strongly recommended.
London is served by: Thames Water, Affinity Water, SES Water. Enter your postcode to identify your exact water company.
London has water companies with active DWI PFAS improvement notices. Enter your postcode to check your zone.