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The maths behind the answer
Everything you have done before the final is worth whatever percentage the exam is not. If the final counts for twenty percent of your grade, the rest of the semester accounts for the other eighty. The calculator takes your current grade, works out how much of your target that eighty percent already delivers, and then asks the exam to cover whatever is still missing.
This is why a heavily weighted final is both a threat and an opportunity. When the exam is worth half the grade, a poor semester can still be rescued, but a strong semester can also be undone. When it is worth ten percent, almost nothing you do on the day will move your grade by more than a point or two, which is worth knowing before you lose a night of sleep over it.
When the number comes back above 100
Sometimes the answer is simply out of reach, and the honest thing a calculator can do is say so. If you need 112 percent on a final that has no extra credit, no amount of studying will get you there. The useful move at that point is to lower the target by one grade band and run it again, which usually returns something achievable, and to ask your teacher whether any extra credit or a dropped lowest score exists.
It also works the other way. Students are often surprised to find their grade is already locked in and the final can only confirm it. If the number comes back at zero or below, you have banked the grade you wanted. Sit the exam properly anyway, but you can stop panicking.
Getting your inputs right
The result is only as good as the current grade you feed it, so pull that from your school portal rather than guessing. If your portal shows a letter instead of a percentage, the dropdown next to each box fills in a sensible mid-band figure for you. The exam weight comes from the syllabus, and if your final replaces a lower test score rather than sitting on top of the others, the calculation here will not match what your teacher does, so check that rule first. To rebuild your current grade from scratch, use the grade calculator.