As per its original definition, the Carnegie Unit is 120 hours of class or contact time with an instructor over the course of a year at the secondary (American high school) level.
Further complicating the computation is the fact that American schools typically meet 180 days, or 36 academic weeks, a year.
These units came about during the late 19th and early 20th centuries through a series of three disjointed events, all designed to standardize the collegiate educational experience.
These processes varied greatly among U.S. colleges and universities, due to the highly subjective nature of these types of examination.
As part of their framework, the Carnegie Foundation also established that both high school preparation and college "work" would include a minimum of four years of study.
On a parallel track, the Carnegie Foundation also underwrote the work of Morris L. Cooke's "Academic and Industrial Efficiency."
Then to create equity in student hours, adjustments are made either to the length of class time or to the assignment of course credit.
As higher education is undertaken by the "masses" in the United States, these tools provide the ability to manage and compare students, faculty, and institutions.