Parts per Million and Parts per Billion

For very dilute solutions, parts per million (ppm) is a convenient way to express concentration:

where cppm is the concentration in parts per million. The units of mass in the numerator and denominator must agree so that they cancel. For even more dilute solutions, 10⁹ ppb rather than 10⁶ ppm is used in the previous equation to give the results in parts per billion (ppb). The term parts per thousand (ppt) is also used, especially in oceanography.