When evaluating the amount of a work, courts evaluate both the quantity and quality of the copyrighted material that was used. If the use includes a large portion of the copyrighted work, fair use is less likely to be found; if the use employs only a small amount of copyrighted material, fair use is more likely.

Furthermore there is the substance of the work or the "heart" of the work. This is a subjective criterion, but the general idea is that if you take the most characteristic elements of a work, even if very it's brief (e.g. "ET phone home") this could result in infringement.

When evaluating the third criterion the amount of the work copied can weigh in favor of fair use if it is:

Relatively small amount (e.g. approximately 10% or 1 chapter, whichever is less)
Not central to entire work (i.e. the most characteristic or climactic elements of a work)

Factors weighing against Fair Use under the third criterion include if the work is:

A large portion or entirety of a work,
Portion used is central to the work

Case Studies Decisions

Fair Use Found:

De Fontbrune v. Wofsy

Issue: Whether reproduction of photographs documenting artwork in a reference book is a fair use.

The United States District Court for the Northern District of California observed that "the amount and substantiality of the work used, weighed in favor of fair use because Defendants copied 'less than ten percent' of the catalogue and there was no evidence the copied images constituted the heart of the work."

Fair Use not Found:

Code Revision Comm. v. Public.Resource.Org, Inc (Links to an external site.)

Issue: Whether it is a fair use to make available for free online a verbatim copy of a state’s official annotations to its statutory provisions without obtaining authorization from the copyright owner.

United States District Court for the Northern District of Georgia "found that the third factor, the amount of work used, weighed against fair use because defendant 'misappropriated every single word of every annotation using a bulk industrial electronic scanner.'"