Fair use is how we keep copyright laws from preventing beneficial uses of copyrighted works, like for news reporting commentary, criticism, research, and educational uses. In deciding whether or not a use is fair, Congress has directed courts to consider, at a minimum, four factors. These four factors are 1) the purpose and character of the use, 2) the nature of the work being copied, 3) the amount and substantiality of the copied material, and 4) the effect of the use on the market for the original work.
The first factor, sometimes also called the “transformativeness” factor, focuses on the new use. Quoting a work for the purposes of highlighting an important insight or observation (say, in a tweet or a Tumblr reblog), would be one example of a use that satisfies this factor, as would quoting for the purpose of rebutting or responding to the work. Of course, you’re not limited to quoting. Parodying a work is also considered a transformative use, such as the infamous “Naked Gun 33⅓” movie poster featuring a ‘pregnant’ and nude Leslie Nielsen, a direct reference to a notorious Vanity Fair cover featuring a pregnant and nude Demi Moore.
The second factor focuses on nature of the original work. Courts are more likely to find that a use is fair if the original work is already published, rather than unpublished, like a set of private letters, or a yet to be released magazine interview. Similarly, courts are more likely to find that a use is fair where the original work is factual in nature, like a news report, as opposed to an original fictional work.
The third factor considers how much of the original work is being taken, and how substantial that portion is. This factor is often summarized as “did you take only as much as you needed to make your use?” There’s no specific formula for determining how much of a work you can quote before it’s “too much.” It really depends on what your use is, and how much of the original work there is. If you’re debunking the factual claims in a lengthy article, it makes sense to quote the claims, and as much additional context as necessary, but not the whole article. But if it’s a really short article, maybe just a paragraph, then it may be necessary to quote the entire content of the original work in order to achieve your purpose.
The final factor asks whether the new work will have an impact on the market for the original work. This is often framed as “does the new use create a substitute for the original work?” It’s important to note that impacting the market for the original work because you criticized it isn’t an issue – it’s whether your work impacts the market by acting as a substitute for the original. That’s not always fatal though – again, think about quoting the entirety of a short work, like a poem, in order to analyze its meaning. Even though the entire poem is quoted within your critique, and someone could read the entire poem without finding a copy made by the author, the transformativeness of the use would probably make it fair.