How do I know if the water in a mains system is hard or soft?
Water hardness is caused by dissolved calcium and magnesium. The hardness of water supply in the UK will very depending on location. The hardness of water, expressed in mg/L CaCO3 (calcium carbonate), can be classified as shown below:
| Water Hardness (mg/L CaCO3) | |
| Soft | up to 100 |
| Slightly hard | 100 -150 |
| Moderately hard | 150 – 200 |
| Hard | 200 – 300 |
| Very hard | More than 300 |
