I have a GPA from a single school and I want to add it into my cumulative GPA.
A cumulative GPA is the final GPA from a single university, college, or school, not from all of them.
While it may be tempting to "raise" the GPA from a particular university by incorporating better results from another school, you will also be lowering the GPA from the school where you did better.
Admissions officers or committees are interested in how you did at each separate school, particularly if you can show increasing cumulative GPAs over time.
Generally, students' computation or renormalization of their own GPAs is viewed as at-best-irrelevant by most interested parties, or, worse, as deception or manipulation. Let your transcripts speak for themselves. - paul garrett
When I apply at universities, they want me to give them "All transferable undergraduate college work: GPA".
So, you want to know how they expect you to calculate an overall GPA, taking into account courses taken at more than one institution, right?
That being said, I can tell you how to do a homemade calculation, for your own purposes. (If you already know how to do this, please ignore the following.) For each course you've taken, multiple the credit hours by the numeric grade (4 for A, 3 for B, and so on). Add up these "weighted" numeric grades for all the schools you've attended to get the grand total. Now to calculate the average, you'll divide that grand total by the total number of credit hours.