Ask your school's registrar office how your school calculates credit hours. While U.S. colleges generally use the Carnegie credit system, it is not uncommon for them to use a slight variation. Examples of variations include using a 15-week semester, instead of a 16-week semester, and basing 1 credit unit on 50 minutes of class time, rather than a full hour.
Calculate how many hours you spend in class per week. Remember that some schools use a 50-minute class hour. These are hours spent in lectures or seminars. If your class meets three times a week, you have 3 class hours.
Calculate the number of preparation hours you get credit for, based on the number of classroom hours. For each hour spent in the classroom, students are also expected to spend an additional 2 equivalent hours a week learning outside of the classroom. If your class meets three times a week for about an hour, you have 3 class hours and 6 preparation hours for the week.
Calculate the length of the semester. The Carnegie credit system is based on a 16-week semester, but schools often use a variation of 15 weeks. To get the credit, students must achieve the minimum amount of hours based on a 15- or 16-week unit. One class credit is equal to 1 hour of classroom time and 2 hours of preparation time over 15 to 16 weeks. Using this calculation, a 3-credit course meets three times a week for 15 to 16 weeks, and has 6 hours of preparation per week for 15 to 16 weeks. The total hours for this 3-credit course is 45 to 48 class hours and 90 to 96 preparation hours, for a grand total of 135 to 144 hours.