ppm is the abbreviation for parts per million, a measure often used in very small concentrations of gasses. It is also used in specifying the accuracy of the crystals used in electronic devices, and in many situations when the percentage is much less than 1%.

Calculation of ppm is simple for anyone who can calculate a percentage; the procedure is the same, except you multiply by 10^6 (one million) rather than 10^2 (one hundred). This also means you can convert from a percentage to ppm by multiplying by 10^4 (ten thousand), and convert back from ppm to percentage by dividing by the same amount

For example, marijuana can have a THC concentration of between 5 and 20 percent. These percentages are large and easy to work with. Hemp-based edible oils, on the other hand, have THC concentrations of only about 0.005%. Since exponential notation looks a bit silly next to a percentage sign, the best way to get rid of those zeros to the right of the decimal point is to use ppm. Simply multiplying 0.005 by 10000 gives us 50 ppm. Easy!