American students are assigned letter-grades depending on their actual percentage grade or success in a class. The highest letter-grade, A, denotes excellence in a class, usually 90 to 100 percent and it is worth a 4.0 on the GPA scale. B represents an average grade in the 80 percent-range, or a 3.0 GPA. Students with C-grades have achieved between 70 and 79 percent and receive a 2.0 GPA, while D-students are struggling, typically at 50 to 59 percent at a 1.0 GPA. Students achieving below 50 percent are graded with an F, which is worth 0.0 on the GPA scale.
Your courses will carry different weights when it comes to your GPA. Generally, to find a given course's GPA weight, you must convert your letter grade into a GPA value, then multiply it by the course weight. For some schools this would be the number of credits offered in the course, or an assigned weight, usually one for a full course or 0.5 for a half-course. Once your courses are weighted, add the values together and divide by the total weight of all your courses to find your GPA.
Grade point average, especially one calculated cumulatively over the course of study at an institution, is a more accurate representation of your progress and learning than tests or exams. Your GPA represents your work over time, rather than the results of a single test, on which you could have scored unusually high or low for any number of reasons. This is why colleges consider your GPA over the course of your high school career -- rather than the results of your finals from senior year.
Your GPA reflects how well you did at your school, but grading can be subjective from school to school and even from teacher to teacher. A 3.5 GPA at a particularly high-achieving and difficult school may require more effort to achieve than a 4.0 at another school. A high GPA at any school however, requires consistent effort and is therefore more representative than singular test results, which is why it is the standard academic measure for college admissions.