Each factor must be weighed in determining if Fair Use applies:
The Purpose and Character of your Use: Many uses that are non-profit and educational are favored under the copyright law and are fair uses, although not all non-profit, educational uses are fair. Note that if your use transforms the original work in some substantial way, in effect creating a new work, that will also support a fair use argument.
The Nature of the Copyrighted Work: The nature of the work is an evaluation of the type and kind of work you wish to use. Use of works that are more factual in nature (such as news or journal articles, formulas, scientific theories) would be more likely to be considered fair use. Also, consider the currency of the work used. A very popular work that is currently making revenue for the rights holders (such as a Hollywood blockbuster) is given greater protection and its use would more likely be unfair.
The Amount and Substantiality of the Portion Taken: The amount of material used is related to the source as a whole. The larger amount of material that is used, the less the court may find the use to be fair. Even a small amount could be considered too much if it is the “heart of the work". There are times and circumstances in which using an entire work might be fair.
The Effect of the Use Upon the Market of the Work: If the work is used to avoid a purchase of the work, the use is likely unfair.
Need more help? Check out the Fair Use for Non-fiction Authors scenarios from the Authors Alliance.