What's a ppm?

Parts per million (ppm) is a unit for measuring the concentration of a particular substance in water. The ppm value will indicate the parts (by weight) of a substance in relation to one million parts of water. One ppm is equivalent to one gram in one million grams. For example, if you had one million marbles and all of them were green except for one red one, the concentration of red marbles would be 1 ppm. If you had 15 red marbles, the concentration would be 15 ppm.

For water, 1 ppm is equivalent to 1 milligram/liter (mg/L). Some test kits have you record the concentration of a particular chemical in mg/L instead of ppm, and it is fine to provide your data with those units.