The actual yield is the mass of product you really collect at the end of an experiment. It is usually less than the theoretical yield, which is the most the equation says you could make. The percentage yield compares the two.
Percentage yield = actual yield ÷ theoretical yield × 100.
Example: a reaction has a theoretical yield of 20 g and 15 g is collected. Percentage yield = 15 ÷ 20 × 100 = 75%. Another example: a student expects 25 g of product but collects 20 g, so the percentage yield is 20 ÷ 25 × 100 = 80%.
The yield is lower than 100% for several reasons. A reversible reaction does not go to completion. Some reactants may take part in side reactions that make other products. Some product is lost when it is filtered, transferred between containers or left stuck to apparatus. The actual yield cannot really be above the theoretical yield, so a value over 100% means the product is impure or still wet.