Every grade point calculator does the same arithmetic: grade points multiplied by credit hours, summed, divided by total credit hours. What separates a correct answer from a wrong one is the table of grade points it uses, and almost every general calculator on the internet uses a table nobody published.
The two inputs that decide the answer
Your own institution's grade point values, and the credit hours of each course. Both are on the transcript and the first is on the registrar's page. A calculator that assumes A- is 3.7 will be wrong at Nebraska, which prints 3.67, wrong at Cincinnati, which prints 3.6667, and badly wrong at Memphis, which prints 3.84. Over a full degree those differences are visible in the second decimal place, which is where honours are decided.
Term, cumulative and projected
A term calculator averages one period. A cumulative calculator averages everything, which means it needs your existing total quality points and total credit hours, not just your existing average. A projection calculator asks what a coming term would do to the cumulative figure, and its honest output is a range, because it depends on how many credit hours you are taking as much as on the grades.
High school and graduate calculators
A high-school calculator has to know whether the scale is weighted and by how much, which is a school policy rather than a national one. A graduate calculator usually works on a shorter transcript with heavier courses, so each grade moves the average much further. Neither is a different formula; both are the same weighted mean with a different table and a different denominator.
Questions people ask about gpa calulator
What does a GPA calculator need to know?
The grade points your institution assigns to each letter, and the credit hours of each course. Without the first it is guessing, and the tables in this index differ from one another in the second decimal place.
Why do two calculators give me different answers?
Because they are using different grade point tables. A calculator built on 3.7 for an A- will not match one built on 3.67, and neither will match an institution printing 3.84.
Can a calculator predict my final GPA?
It can project one, given your current quality points, your current credit hours and the credit hours you intend to take. Treat the output as a range rather than a figure, because the denominator moves as well as the numerator.