The optimum range of hardness in drinking water is from 80 to 100 mg/L. Water with hardness greater than 200 mg/L is considered poor and water with hardness greater than 500 mg/L is normally considered unacceptable for domestic use.

Most water hardness is naturally occurring from weathering of rocks and minerals. It affects taste but not health. It may form deposits in water tanks, pipes and water heaters.