Carnegie Units - 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 typical public school and private school classes usually meet for 50 minutes or less means it takes more time to earn credits. Not all schools are correctly calculating Carnegie Units.
1 Carnegie Unit = 1 credit = 120 hours (7200 minutes) study in one subject.
DaVinci Learning Academy
Flexible year round scheduling = 5 hours per week per class, scheduled in a way that works for you. (Typically 5, one hour classes.) Each quarter is scheduled for 13 weeks, with one flex week built in for time off.
US Public Schools
Rigid August - June scheduling for 180 days. Typically meet 5 days week w/ 40 minute classes = 3 hours 20 minutes per week per class. Vacation days are mandated by the schools schedule. No flexibility.
Other Methods of Calculating Credits
Basically, you have to reach the correct number of student work hours. Carnegie Units don't tell you how you have to earn them, just the amount of student work hours.
5 days/week * 1 hour a day for 24 weeks = 7200 minutes = 1 credit
4 days/week * 1 hour a day for 30 weeks = 7200 minutes = 1 credit
3 days/week * 1 hour a day for 40 weeks = 7200 minutes = 1 credit
60 hours = 3600 minutes = .5 credit