Each peroxy group is considered to contain one active oxygen atom. The concept of active oxygen content is useful for comparing the relative concentration of peroxy groups in formulations, which is related to the energy content. In general, energy content increases with active oxygen content, and thus the higher the molecular weight of the organic groups, the lower the energy content and, usually, the lower the hazard.
The term active oxygen is used to specify the amount of peroxide present in any organic peroxide formulation. One of the oxygen atoms in each peroxide group is considered "active". The theoretical amount of active oxygen can be described by the following equation:
where  is the number of peroxide groups in the molecule, and  is the molecular mass of the pure peroxide.
Organic peroxides are often sold as formulations that include one or more phlegmatizing agents. That is, for safety sake or performance benefits the properties of an organic peroxide formulation are commonly modified by the use of additives to phlegmatize (desensitize), stabilize, or otherwise enhance the organic peroxide for commercial use. Commercial formulations occasionally consist of mixtures of organic peroxides, which may or may not be phlegmatized.