Two different classes can grade the exact same scores completely differently, depending on which system they use.
Every assignment is worth a certain number of raw points, and your final grade is simply everything you earned divided by everything that was possible:
Final grade = total points earned ÷ total points possible
In this system, a 100-point final exam automatically counts more toward your grade than a 10-point homework assignment — just because it's worth more raw points.
Assignments are grouped into categories (Homework, Exams, etc.), and each category is assigned a fixed percentage of your final grade — regardless of how many raw points it contains:
Final grade = (weight × score for each category, added up) ÷ total weight
In this system, a category worth 40% of your grade counts as 40%, whether it contains one 100-point exam or ten 10-point quizzes.
Say a class has 900 points of homework and one 100-point final exam (1,000 points total). Under points-based grading, the final exam is worth only 10% of the grade, since it's just 100 of the 1,000 total points — homework dominates.
Now say that same class instead weights Homework at 30% and the Final at 70%, regardless of point totals. Under weighted grading, the final now counts for 70% of your grade — a huge difference from the 10% it was worth under the points-based version, even though nothing about the actual assignments changed.
This is why it matters to know which system your class actually uses before you estimate your grade.
The Grade Calculator supports both directly — switch between "Weighted Categories" and "Total Points" mode depending on which one matches your class.