Water hardness is most commonly caused by the amount of dissolved calcium and magnesium. In most waters, these elements dominate completely, so the others can be neglected and the exact definition is not that important. Calcium contributes more to water hardness than magnesium (the Ca:Mg ratio in most natural waters is 2:1 to 4:1). In nature, water can be found as very soft or very hard — it all depends on which minerals the water has come into contact with. Tap water is only rarely very soft; it is more often moderately to very hard. The harder the pool water is, the more limescale builds up on pool walls.

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