The formula
needed = (target − current × (1 − w)) ÷ w
where w is the final's weight as a decimal and current is your grade on everything else so far. The term current × (1 − w) is the part of the course you have already banked; the rest has to come from the exam.
A worked example
You have 82%, the final is worth 25%, and you want an overall 88%.
- Already banked: 82 × 0.75 = 61.5 points of the 100.
- Still needed: 88 − 61.5 = 26.5 points.
- From an exam worth 25 points: 26.5 ÷ 0.25 = 106%.
So an 88 is not reachable. The best possible grade is what you get with a perfect exam: 61.5 + 25 = 86.5%.
Two numbers worth having
The best you can still get: current × (1 − w) + 100 × w. Nothing above that is available at any effort.
The worst you can get: current × (1 − w), which is your grade if you score zero. With a lightly weighted final, that floor is often still a pass, which tells you how much the exam actually matters.
Get the current grade right first
This only works if current is the weighted average of everything that is not the final, scaled to the same 0–100 as the target. Two things commonly break it:
- Categories with their own weights. Work out the weighted average of those categories first, then use it here.
- A dropped lowest score, or a curve. Those change the current figure, so apply them before this step rather than after.
When the number comes out impossible
Above 100% means the grade is out of reach with the exam alone, and the honest options are extra credit, a resit, or aiming at the next grade down. That is worth knowing in week ten rather than after the exam, which is really what this calculation is for.
Tools
The Final Grade Calculator gives the score you need, the best grade still available and the result if you score zero. Work out the current figure with the Weighted Average Calculator, and the GPA Calculator turns the finished letter grades into a GPA.