Add up your marks, divide by how many there are, and you have a number. It is almost never your grade, and it is usually too high.
The reason is that courses do not treat their assessments as equals. A final exam worth half the course and a homework worth a tenth of it are not two things to average — one of them is five times the other. Averaging quietly promotes the small easy marks to the same importance as the big hard one, which is exactly the wrong direction.
How weighting actually works
Multiply each mark by the share of the course it is worth, then add the results. That is the whole method.
With the marks above: 100 × 0.10 is 10, plus 95 × 0.10 is 9.5, plus 65 × 0.30 is 19.5, plus 70 × 0.50 is 35. Total 74.
The useful way to think about it is points banked out of points available. Each component is a pot of points the size of its weight, and your mark says what fraction of that pot you collected — the same percentage arithmetic as anywhere else, just applied four times and added up. A perfect homework is worth ten points because the homework is worth ten per cent — no more, however good it was.
Which explains the thing that frustrates students most: an excellent mark on something small barely moves the total. It is not unfair, it is arithmetic, and it was written on the syllabus in week one.
What do I need on the final?
This is the question everybody actually has, and it is the same equation read backwards.
Work out the points already banked, subtract that from your target, and divide by the weight of what is left. With the marks above, before the final: 10 plus 9.5 plus 19.5 is 39 points banked, out of the 50 available so far. The final is worth 50.
- For a 70 overall: 70 − 39 = 31, divided by 0.50, is 62 on the final.
- For an 80: 80 − 39 = 41, divided by 0.50, is 82.
- For a 90: 90 − 39 = 51, divided by 0.50, is 102. Which does not exist.
That last line is worth as much as the other two. A perfect final here gives 39 plus 50, which is 89 — so 90 stopped being available before the exam was sat. Knowing that a week early is genuinely useful information, and it is why a calculator that says so is better than one that prints 102 without comment.
The rules that change the arithmetic
Several common syllabus clauses break the simple sum, and all of them are in your favour if you read them:
- Dropping the lowest. If the lowest of six quizzes is dropped, the pot is divided among five, so each surviving quiz is worth more than the syllabus percentage divided by six.
- The final replacing the midterm if it is higher. Common, and it means a bad midterm is sometimes not the disaster it looks like.
- Categories with unequal items inside them. "Assignments 30%" across four assignments is 7.5% each — unless the syllabus says one of them counts double.
- Extra credit, which usually adds points to the total rather than to a category, and therefore has a different effect on the sum entirely.
None of these can be guessed at. They are on the syllabus, and reading it once at the start is worth more than any calculator.
Where rounding decides things
A boundary at 80 makes 79.5 either an A− or a B+, depending on a rule the syllabus may or may not state. Some schemes round to the nearest whole number, some truncate, some round only at the letter boundary, and some do not round at all.
If you are within half a point of a boundary, that rule is the single most valuable thing you can find out, and it is a fair question to ask.
There is a wider habit here worth having. Percentages combine in ways that feel wrong until you have seen it once — the reason averaging weights overstates your grade is the same reason 20% off and then 10% off is not 30% off. Our guide on percentages walks through several of those, and they all come from taking a percentage of the wrong base.
A calculator is not the authority
Everything here describes the commonest scheme, and yours is whatever your syllabus says. Points-based courses, courses that curve to a distribution, courses with a hurdle you must pass regardless of the total — all of these exist, and none of them is what a weighted average computes.
So use the number to decide where to put your effort, which is what it is good for, and use the syllabus to decide what your grade is.