The grade point average formula
A GPA is a credit-weighted average of grade points. Every letter grade converts to points on the 4.0 scale: A is 4.0, A- is 3.7, B+ is 3.3, B is 3.0, B- is 2.7, C+ is 2.3, C is 2.0, C- is 1.7, D+ is 1.3, D is 1.0, D- is 0.7 and F is 0. Multiply each course's points by its credit hours to get quality points, add the quality points, then divide by the total credit hours. A four-credit course influences the result twice as much as a two-credit course, which is why one bad grade in a lab-heavy science hurts more than the same grade in a one-credit seminar.
Worked example: Calculus, four credits, A- gives 3.7 times 4, or 14.8 quality points. English, three credits, B+ gives 3.3 times 3, or 9.9. Chemistry, four credits, A gives 4.0 times 4, or 16.0. Total quality points are 40.7 across 11 credits, so the semester GPA is 40.7 divided by 11, which is 3.70.
Cumulative GPA and weighted GPA
Your cumulative GPA is the same formula applied to every course you have ever taken, not the average of your semester GPAs. To update it, convert the old record back into quality points by multiplying the prior GPA by the prior credits, add this semester's quality points, and divide by the combined credits. Continuing the example, a student with a 3.50 cumulative GPA over 30 credits carries 105.0 quality points. Adding 40.7 points and 11 credits gives 145.7 over 41 credits, a new cumulative GPA of 3.55. The more credits are already on the transcript, the harder it becomes to move the number, which is why freshman grades matter disproportionately.
Many high schools publish a weighted GPA that adds 0.5 for honors classes and 1.0 for AP or IB classes, so an A in AP Biology counts as 5.0 and the overall average can exceed 4.0. There is no national standard: some districts cap the weighted scale at 5.0, some weight only AP, and colleges routinely recalculate applicants' GPAs using their own unweighted method and only core academic subjects.
Scale variations to watch for
Check your school's catalog before trusting any calculator, including this one. Some institutions award 4.3 for an A+, some treat A+ and A identically as 4.0, and some do not use plus and minus grades at all. Pass/fail courses, transfer credits, audits and withdrawals usually carry credit but no quality points, so they are excluded from the average. Repeated courses may replace the original grade or average with it depending on the forgiveness policy. Graduate programs commonly require a 3.0 to stay in good standing, while undergraduate academic probation often begins below 2.0.