My water supply

Water hardness

When water falls as rain, it's "soft" and free of minerals.

It picks up naturally occurring minerals, such as calcium and magnesium, as it passes through rock, sand and soil. This causes the water to become ‘hard.’

All the water we supply is ‘hard’ together with more than 60% of homes in the UK.

The picture shows a comparison with other areas in the UK. 


Water Hardness in the UK

Hard water in your home

Find out more about hard water in your home

Find out how hard the water is in your home

1. Find the water quality zone for your area

2. Use the water quality zone to read the table below:

Zone Zone Name Alkalinty (Average mg/l) Calcium
(Average mg/l)

Calcium CACO3 (Average mg/l)

Magnesium (Average mg/l) Magnesium CaCO3 (Average mg/l) Total Hardness
mg/l or ppm CaCO3
Degrees Clark Degrees French Degrees German
Z1 Cambridge North 293 112 280 4.16 17.1 297 20.8 29.7 16.7
Z2 Cambridge South 305 120 300 4.66 16.4 316 22.1 31.6 17.8
Z4 Linton 337 117 293 5.6 23 318 22.3 31.8 17.9
Z5 Heydon 288 100 250 3.95 16.3 266 18.6 26.6 14.9
Z6 Coton 297 108 270 4.3 17.7 294 20.6 29.4 16.5
Z7 Sawston 299 110 275 7.26 29.9 306 21.4 30.6 17.2
Z8 Odsey 330 98.5 246 3.4 14 260 18.2 26 14.6
Z10 Earith Bridge 274 115 287 2.4 9.9 297 20.8 29.7 16.7