Hard water is caused by excessive levels of calcium and/or magnesium dissolved in water. The U S Department of Interior classifies hardness based on the concentration of calcium and/or magnesium as measured in grains per gallon (gpg). To put this in perspective, a typical aspirin weighs about 5 grains (1 grain = 1/7000 pound). If the aspirin were dissolved in a gallon of water it would add 5 grains of aspirin to the water.