In theory, the maximum percent yield of any reaction would be 100 percent. In chemistry, the formula used to find percent yield is dividing the actual yield by the theoretical yield and then multiplying this value by 100 percent.
In this formula, the theoretical yield is the amount of the product formed if the laboratory reaction went to an assumed perfect completion, according to Organic Chemistry website from the University of Colorado. The actual yield is how much of the product really forms in a laboratory reaction. However, in most laboratory reactions, the actual yield is typically less than the theoretical yield. For this reason, percent yield is less than 100 percent.
In a reaction, the actual yield may be less than the theoretical yield due to product losses through processes like filtration or product transfer. Some other possible causes for the reduction in the percent yield are impure reactants, errors in measurement of the resulting product and reactants forming other unwanted products.