The amount of product that may be produced by a reaction under specified conditions, as calculated per the stoichiometry of an appropriately balanced chemical equation, is called the theoretical yield of the reaction. In practice, the amount of product obtained is called the actual yield, and it is often less than the theoretical yield for some reason. Some reactions are inherently inefficient, being accompanied by side reactions that generate other products. Others are, by nature, incomplete. Some products are difficult to collect without some loss.

The extent to which a reaction’s theoretical yield achieved to expressed as its percent yield:

Percent yield = (actual yield ÷ theoretical yield) × 100%

Actual and theoretical yields may express as masses or molar amounts (or any other appropriate property, e.g., volume, if the product is a gas). As long as both yields expressed using the same units, these units will cancel when the percent yield calculated.