The Four Factors can be understood as weights on a balance scale, one side which favors fair use and the other which opposes. An analysis of the factors places a small or large weight in one pan or the other depending upon the determination. Once you consider all four factors, how balanced the scale is will help determine if the use favors fair use or not.
Is your use absolutely protected by the Fair Use Provision? There is no definitive answer. (Not that people haven’t tried.) A reasonable evaluation of the Four Factors comes when you learn about them and understand how courts have ruled about them in cases.
This Fair Use Analysis Tool will help you see how the four factors weigh with and against each other to help you decide what is fair use.
You may also get assistance from a copyright specialist or AU's Office of General Counsel.
Fair Use Guidelines
There are few bright lines between legal and illegal no matter what some might say.
Be aware that guidelines you might use or read about are merely suggestions. US Copyright Law offers no quantities for assessing fair use. Guidelines may help you to think about amounts, but may severely limit your ability to use or reuse copyrighted works in reasonable ways that exceed the guidelines but still fall within the provisions of Fair Use.
In copyright, there are very few true bright lines. Ultimately, only a court can determine if a use is fair. Statutory law can only say so much, then a judge must interpret through case law.