Hardness is defined as the concentrations of calcium and magnesium ions expressed in terms of calcium carbonate, which can be calculated as shown in the equation:

Hardness mg/l = 2,5 [conc. Of Ca²+ (mg/l)] + 4,1 [conc. Of Mg²+ (mg/l)]

The most frequently used standard classifies water supplies is show in Table 1:

These minerals in water can cause some everyday problems. They react with soap and produce a deposit called "soap curd" that remains on the skin and clothes and, because it is insoluble and sticky, cannot be removed by rinsing. Soap curd changes the pH of the skin and may cause infection and irritation. It also remains on the hair making it dull and difficult to manage. Soap curd picks up the dirt from laundry water and holds it on cloth, contributing to a gray appearance of white clothes. It is especially troublesome when wash water is allowed to drain through the clothes. The use of synthetic detergents may help a little, but the active ingredient in the detergent is partially inactivated by hardness and more detergent must be used for the same cleaning task. Some detergents will produce soap during the reaction with oil or grease on the surface being cleaned and as a result they will also deposit soap curd. A ring around the bathtub and spotting on glassware, chrome and sinks are constant problems in the presence of hard water. They require additional rinsing and wiping, increasing the time spent on everyday cleaning.

Cooking with hard water can also be difficult, producing scale on pots. Some vegetables cooked in hard water lose colour and flavour. Beans and peas become tough and shriveled.

Hard water may also shorten the life of plumbing and water heaters. When water containing calcium carbonate is heated, a hard scale is formed that can plug pipes and coat heating elements. Scale is also a poor heat conductor. With increased deposits on the unit, heat is not transmitted to the water fast enough and overheating of the metal causes failure. Build-up of deposits will also reduce the efficiency of the heating unit, increasing the cost of fuel.

Most natural water supplies contain at least some hardness due to dissolved calcium and magnesium salts. Other minerals, such as iron, may contribute to the hardness of water, but in natural water they are generally present in insignificant quantities. The total hardness of water may range from trace amounts to hundreds of milligrams per liter.