Abbreviated as ppm. It is a "weight-to-weight" expression. It means one part in one million parts, such as, 1 pound of chlorine in one million pounds of water or 119,904 gallons. Many of the common pool water tests as well as acceptable ranges are stated as ppm. For instance, Free Available Chlorine should be kept between 1.0 and 3.0 ppm; Total Alkalinity should be between 80 and 120 ppm; and Water Hardness should be between 200 and 400 ppm. A ppm and a milligram per liter (mg/L) are considered equal.