Here are the four factors specified in the copyright statute to use in determining fair use:
- The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- The nature of the copyrighted work;
- The amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- The effect of the use upon the potential market for or value of the copyrighted work.
(from Section 107 of the U.S. Copyright Law, which permits other factors)
Read how courts have interpreted these provisions and see other factors they consider on the BookContracts Cases Page.