Hard water can be an annoyance in a home due to the build-up of limescale, issues with soap and damage to piping and kettles that is caused by the minerals in the water, which is why people use block salt softeners to reduce the strain on piping.

It comes from where water passes through chalk, limestone and other rock and picks up primarily calcium and magnesium. This creates both carbonate (temporary) and non-carbonate (permanent) hardness, the former of which creates limescale whilst the latter does not.

Measuring water hardness is about measuring the amount of calcium carbonate, which according to United Utilities is typically measured in parts per million (ppm) or milligrams of calcium carbonate per litre of water (mg-CaCO3/L), although there is a range of different measurements that are also used.

The hardest water in the world, unsurprisingly, is seawater, which contains a range of salts and has roughly 6,630 ppm. Water hardness is generally measured between 15 and 375ppm depending on where water is stored and how it travels.

Here are the general classifications for hard and soft water in the UK:

  • 0 – 50 ppm: Soft Water
  • 51 – 100 ppm: Moderately Soft
  • 101 – 150 ppm: Slightly Hard
  • 151 – 200 ppm: Moderately Hard
  • 201 – 300 ppm: Hard
  • 301+: Very Hard


How hard water is in practice can vary, depending on the availability of reservoir water compared to groundwater. This is why, despite being in an area typified by hard water, Manchester’s water is considered to be very soft, due to the reservoirs built in the Lake District.

London, on the other hand, gets most of its water from the River Thames, which is especially hard, measuring at around 275 ppm.