Hard water is water that has a high mineral content. This content usually consists of high levels of metal ions, mainly calcium and magnesium in the form of carbonates. It is estimated that more than 85% of the water used by consumers in the US can be classified at some level of "hard" water. The term "hard water" was originally coined to refer to water that was difficult or hard to work with. The term "hard water" was originally coined to refer to water that was difficult or hard to work with. Hard water requires much more soap, shampoo, or detergent than soft water; and the minerals in hard water can decrease soap's lathering capabilities. A common distinction is made between 'temporary' and 'permanent' hardness.

There are also common types of hard water depending on the ion found in the water. Hard water causes scaling, which is the precipitation of minerals to form a deposit called limescale. Scale can clog pipes and can decrease the life of toilet flushing units. It can coat the inside of tea and coffee pots, and clog and ruin water heaters. Similarly, the insoluble salts that get left behind from using regular shampoo in hard water tend to leave hair rougher and harder to detangle. In industry, hard water contributes to scaling in boilers, cooling towers and other industrial equipment. Hardness is controlled by addition of chemicals and by large-scale softening with zeolite resins and ion exchange resins.Washing hair in soft water will have a different result because it leaves fewer insoluble deposits on the hair.

Hard water tends to make the scales stand up, which makes your hair feel rough and tangly. Types of hard water a common distinction is made between 'temporary' and 'permanent' hardness. Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates. Chlorides in the water, which become more soluble as the temperature rises. Hard water may be treated to reduce the effects of scaling and to make it more suitable for laundry and bathing. Soft water lathers better than hard water but leaves a "slippery feeling" on the skin after use with soap.