A grade point average is a weighted mean. Multiply each course's grade points by its credit hours, add those products, and divide by the total credit hours. Everything difficult about it is in the first step, because the grade points come from your own institution's published table and the tables do not agree with each other.
The arithmetic, in one line
Sum of (grade points x credit hours), divided by sum of credit hours. Oregon State states its table as grade points per credit rather than as a bare lookup, which is the same formula written as a unit: a three-credit A contributes twelve quality points and three credit hours, a one-credit A contributes four and one. That weighting is why a heavy course carries the average and a one-credit seminar barely moves it.
Where the grade points come from
Your registrar's published table and nowhere else. Indiana prints A- at 3.7 and B+ at 3.3. Nebraska prints 3.67 and 3.33. Cincinnati prints 3.6667 and 3.3333. Memphis prints A- at 3.84. South Carolina prints B+ at 3.5 and has no minus grades at all. Using a table from a study site rather than your own registrar's is the single commonest way a home calculation comes out wrong.
What the calculation excludes
Pass and fail marks, withdrawals, incompletes and audits usually carry no grade points and no credit hours, so they leave the average untouched rather than counting as zero. Indiana lists P, S, W and audit marks beside its grade table with dashes where the points would be. A failed course is different: it carries zero grade points and its credit hours still count, which is why one F moves an average further than one A.
Questions people ask about how to calculate gpa
How do I calculate my GPA by hand?
Multiply each course's grade points by its credit hours, add the results, and divide by the total credit hours. The grade points come from your own institution's published table, which is the part a general formula cannot supply.
How do I calculate a GPA from percentages?
Only where your institution publishes the mapping. The University of Toronto prints letters, percentage bands and grade points in one table, so a percentage there converts. Most American registrars publish no percentage at all, because the percentage is set course by course.
Do withdrawals and pass grades count?
Usually not. Marks with no grade point and no credit hours leave the average untouched. A failure is the exception: zero grade points, credit hours still counted.