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. Strictly speaking, this breaks down into a single one-hour meeting, on each of five days per week for a total of 24 weeks per year. However, knowing that classes usually meet for 50 minutes yields a value of 30 weeks per year. However, further complicating the computation is the fact that American schools typically meet 180 days, or 36 academic weeks, a year. A semester (one-half of a full year) earns 1/2 a Carnegie Unit.