London Water Hardness: 306 ppm

London has some of the hardest water in the UK, averaging 306 ppm (parts per million calcium carbonate) — classified as very hard. That is roughly 30% above the national average of 230 ppm.

The city is supplied by Thames Water, which draws most of London's supply from chalk aquifers and the River Thames. The underlying geology of the London Basin — Cretaceous chalk beneath London Clay — dissolves calcium and magnesium into the water before it reaches treatment works.

London Water Hardness Level by Postcode

Hardness across London's 171 postcode districts ranges from around 250 ppm in some South West areas to over 350 ppm in parts of East London and the City. The table below shows the exact ppm reading for every London postcode district.

If you live in London and notice limescale in your kettle, white residue on taps, or dry skin after showering, the high mineral content is the direct cause. A water softener or shower filter can make a significant difference.

How London Compares

• London: 306 ppm (very hard)
• UK average: ~230 ppm (hard)
• Manchester: ~40 ppm (soft)
• Edinburgh: ~50 ppm (soft)
• Cardiff: ~65 ppm (slightly hard)

See hardness for all Greater London boroughs or the wider London region.