Calcium hardness is a measure of the calcium (Ca+2) ion concentration in pool water expressed as calcium carbonate (CaCO3). Pool water requires a certain amount of calcium. If the level of calcium is too low (soft water), the pool water will be corrosive and dissolve calcium and other minerals from plaster pool surfaces and metal equipment components. If the calcium level is too high (hard water, supersaturated), it can cause turbid water and deposit calcium carbonate scale on pool surfaces and recirculation equipment, particularly heat exchanging surfaces. The calcium hardness should be maintained between 150 and 1000 ppm as CaCO3 in swimming pools. Ideally the calcium hardness should be between 200 and 400 ppm.

Since calcium ions cannot be effortlessly removed from pool water, it is important to know the calcium substance of the source water (make-up water). If the source water is low in calcium it will be necessary to raise the hardness by adding calcium, from calcium chloride, to prevent corrosive water conditions. If the source water is already high in calcium, it will be necessary to “balance” the water accordingly by adjusting total alkalinity and pH to prevent a condition of supersaturation.