Copyright

(2) the nature of the copyrighted work;

(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and

(4) the effect of the use upon the potential market for or value of the copyrighted work.

U.S. Copyright Act, 17 U.S.C., Section 107
Limitations on Exclusive Rights: Fair Use

The fair use provision may be applied to the use of all copyrighted works, even those in digital form. To determine whether any particular use is fair use, you should conduct a case-by-case analysis based on all four of the factors below.

First Factor: Purpose and Character of the Use

If the purpose of the use is educational, such as classroom instruction and restricted access course websites, for research and scholarship, or for criticism, commentary or news reporting, it is likely “fair use.” If the purpose of the use is for commercial gain or public distribution, then it is likely not “fair use.” However, educational use in and of itself will not assure that your use is a fair use, and not every commercial use will fail as a fair use. Transformative uses, uses that result in the creation of a new work, with a new purpose and different character are favored as fair uses over uses that merely reproduce an original work.        

Second Factor: The Nature of the Copyrighted Work
Factual works, published works and scientific articles that are factual in nature are more likely to be considered available for fair use than are creative, imaginative, artistic, or unpublished works.

Third Factor: Amount and Substantiality of the Portion Used
There are no hard and fast rules concerning how much of a work may be used under fair use. Contrary to popular belief, there is no prescribed set amount (no more than 10% rule) that authorizes use. In general, the smaller the amount used, the more it tends to be favored as fair use unless the portion used is considered to be the most essential part, also known as the "heart of the work." The central question is whether the quantity and value of the materials used are reasonable in relation to the purpose of copying. As a general rule of thumb, only use the portion of the work needed to meet your needs. 

 Fourth Factor: Effect on the Potential Market for or Value of the Work
Generally, the consideration for this factor is whether or not there is some economic harm to the owner of the copyright as a result of your use. Repeated use of the same work, or distribution of multiple copies of a work may affect the market value of the work, and such uses are likely not “fair use.” Likewise, if a license to the work or asking the copyright holder permission to use the work are available options, then use of such works is likely not “fair use.” This factor alone, however, cannot determine whether or not a use is fair. If the first three factors weigh in favor of fair use then market harm should carry less weight even when considering the permissions market, since the market is for permissions that are required. Conversely, if the first three factors are tipping the balance in favor of permission then market harm will carry more weight in the balancing of the factors.