Fair use is the flexible and dynamic exception to copyright law that serves to balance the rights of creators with the public interest in using copyrighted works to advance education, to comment and criticize, and to make new creative content. Its flexibility is often somewhat intimidating or frightening because when considering fair use it is very rare to know with certainty that a use is fair, only that it is more or less likely to be fair. It can be frustrating that the law does not give us any clear answers regarding amounts we can use and know that we are "safe." Fortunately, there are ways of understanding the purpose and function of fair use that can help us feel more confident about evaluations and maybe even come to love fair use for its flexibility.
Fair use is determined by considering four factors of that use. No one factor is determinative; each factor must be considered and weighed. Usually after considering each of the four factors and weighing how much each fact of your particular situation favors or disfavors fair use, you are left with an overall sense that your use is "probably fair" or "probably not fair." Really, only the courts can offer us definitive answers.
How do you propose to use the work? Purposes that favor fair use include education, scholarship, research, news reporting, criticism and commentary. Non-profit purposes also favor fair use. Commercial uses weigh against fair use.
The biggest mistake we see educators making is mistaking their educational context for an educational purpose. If you create a class website or presentation and put a pretty picture on it primarily for decoration or visual interest, this is very different from an image used to provide direct instruction.
Since copyright is designed to protect works of creative expression, the more highly creative the work is, the more fair use is weighed against. This is, of course, subjective. We might say, in general, a novel would be more highly creative than a work of non-fiction, but, of course, there is a huge range of creativity within the broad category of "non-fiction." Unpublished works would also be less likely to qualify for a fair use than published works.
It seems that everyone wants to see a percentage or number of pages that will always be fair. There is no 
such number. The goal of fair use is to make available wide and unpredictable set uses. Could a legislator predict in advance that a future satirist would never need more than ten percent of a work in order to make their point? Hardly!
As a general principle: using less of a work is always more likely to be fair than using more. The smaller the portion used relative to the whole, the more likely the use is to be fair.
It is also true, however, that using an entire work can be and often is a fair use.
Another guiding principle: using only the portion of a work that is absolutely necessary in order to meet the educational (or other fair) purpose you have in mind is more likely to be fair than using more than is necessary. For example, if you are considering copying a 4 page article for your class because the author makes an argument you'd like to discuss but that argument could be well understood by reading just a couple of paragraphs of the article, copying just those paragraphs would more strongly favor fair use than copying the entire article.
The more the portion you want to use represents the "heart of the work," the less likely your use is to be fair. This can be a very difficult one to assess.
The most useful way to think about this factor is to ask if your use could substitute for the original in the marketplace. Would your use substitute for sales either to your students or to anyone else? A confusing piece here is the permissions market. A strong market exists in selling permissions to use content, especially materials like book chapters and journal articles. It can be easy to say "Of course my student won't be subscribing to Professional Journal X, so copying an article certainly doesn't substitute in the market." But it would substitute for that secondary permissions market. At this time we don't have a truly conclusive case law to guide us in thinking about the permissions market, but it seems likely that where there is a viable permissions market for the material you want to use, it would weigh against fair use.
Once you have looked at all of the factors, you can assess if taken as a whole your use seems likely to be fair or likely to be unfair. No single factor is determinative, and you could "strike out" in three categories but have the remaining category weigh so strongly in favor of fair use that, overall, your use is fair. We discuss this in more depth below in "Thinking about Fair Use: Transformativeness."
In 1994, the Supreme Court unanimously found that 2 Live Crew's song "Pretty Woman" did not infringe the copyright in Roy Orbison's song "Oh Pretty Woman," despite the fact that the parody song drew from the "heart" of the work of the original, used large amounts of a creative work, was a commercial use, and had negative effects on the market. It would seem in this case that all four factors would weigh against 2 Live Crew. he court found the use fair, however, because 2 Live Crew's use was transformative. This idea had been present in legal scholarship for some time, but this case (Campbell v. Acuff-Rose Music) cemented the concept. The Court finds that when source work is used in new and unexpected ways, the use is more likely to be fair. Some people view "transformativeness" as a kind of "fifth" fair use factor, while others see it as a way of understanding the interplay between the four factors.
In a sense, we can view "transformation" as the goal of fair use. Fair use exists in order to give citizens the ability to take existing creative works and use them to create new knowledge, understanding, or art. We find that when evaluating fair use, it is helpful to keep "transformativeness" in mind as we look at the picture that our four-factor analysis gives us.
"I'm sure I read somewhere that 10% of a work is always fair!"
"Isn't it okay if I use something one term but then I have to get permission the next term?"
We know from looking at the four factors of fair use that no specific amounts are dictated and there's nothing in there about 'spontaneity' or 'cumulative effect' (all that "one term free" business). So why do so many of us remember hearing these things somewhere along the line? These concepts come out of the Classroom Guidelines. These are a set of negotiated guidelines agreed to by the Association of American Publishers and The Author's League of America following the passage of the 1976 copyright law (which first codified fair use). The Guidelines are an agreement between private parties intended to provide a kind of "safe harbor." That is, the publishers were essentially saying that the guidelines represent, in their opinion, an interpretation of the minimum standards of fair use that they could live with and if educators operated within that "safe harbor" they would not be liable for infringement. The guidelines specifically state that they are meant "to state the minimum and not the maximum standards of fair use under section 107". Despite this, many groups including the American Association of University Professors (AAUP) objected to the Guidelines as they feared they would come to be seen as, in essence, a ceiling rather than a floor. That is, to express the limits of fair use rather than the minimum. This fear has most certainly proven to be well-founded with many institutions enshrining the Guidelines as policy. Certainly the Guidelines are widely conflated with the actual law.
The Guidelines can be helpful, especially when regarded as the "floor" they were intended to be. Remember that there is no real guarantee that the Guidelines do actually represent a "safe harbor" as they do not carry the force of law. But, more importantly, remember that the very nature of fair use is that does not attempt to anticipate all future potential uses and describe in advance if those uses are fair.
This page has been adapted from Copyright Resources at Portland Community College
Nearly any image, video, or audio used in class presentations given face-to-face in a classroom setting is permitted. This broad exception does not apply to copying and distributing text, so actions like photocopying an article might not be allowed.
Portions of Fair Use Fundamentals infographic: Association of Research Libraries CC-BY
Winston Mills-Compton teaches a class in mathematics at the Mfantsipim Boys School in Cape Coast by the World Bank Photo Collection, CC BY-NC-SA 2.0
316/365 - Maple-Bourbon Pumpkin Pie by Dennis Wilkinson, CC BY-NC-SA 2.0
Scale: Michael Brewer & ALA Office of Information Technology Policy, CC BY-NC-SA 3.0