Part per million (ppm) is a measure of concentration of a certain number of parts in a total of one million. It is also expressed as milligrams per liter. It takes 1000 milligrams to equal a gram and 1000 milliliters to equal a liter of water. Thus, 1000 times 1000 equals one million. For example, water with 20ppm nitrate has 20 milligrams of nitrate for each liter of water.