Reading the weights off your syllabus
This page assumes you already know your category weights and just want the arithmetic done properly. If you are not sure whether your course is weighted at all, start at the grade calculator, which covers both models.
Syllabi phrase weights in a few ways that need decoding before anything goes into a calculator. "Exams: 3 @ 15% each" means one category at 45%, or three categories at 15% — either entry gives the same answer as long as you are consistent. "Participation: up to 10%" is a 10% category. "Homework and quizzes: 30% combined" is one category, not two, unless the syllabus splits it later. And a syllabus that lists both point values and percentages is telling you the percentages govern; the points are there to show you how the category is built.
The empty category problem
Here is the case that produces most of the wrong answers on the internet. Take a five-category course: homework 15%, quizzes 15%, labs 20%, midterm 20%, final exam 30%. It is week nine. The final has not happened.
There are two defensible things to do with that 30%, and they are nine points apart.
- Exclude it and renormalise. Divide by the 70% that has been graded: (94×0.15 + 81×0.15 + 88×0.20 + 76×0.20) ÷ 0.70 = 84.4%. This says "here is how you are doing on the work that has been marked".
- Count it as zero. Divide by the full 100%: the same numerator over 1.00 = 59.1%. This says "here is where you land if you never sit the final".
Both are true statements about different questions. This calculator excludes empty categories by default, because mid-semester the first question is the one nearly everyone is asking. If you want the second, enter a zero for the final and watch what happens — it is a useful thing to see once.
The formula
G = Σ(sᵢ × wᵢ) ÷ Σwᵢ. Note the denominator: the sum of the weights you actually entered, not a hard-coded 100. That is what makes renormalisation automatic, and it is what lets you enter a partial syllabus and still get a meaningful number.
If your weights do add up to 100 and every category has a score, the two forms are identical and this note does not matter to you.
Where this stops being reliable
Some courses weight by time rather than by category — first half 40%, second half 60% — and that needs a different model. Some finals replace a category rather than adding to one, which means your current grade changes when the final is entered rather than being averaged with it. Some syllabi drop your lowest score in a category, which you have to apply yourself before entering an average. And a category weight that appears in the syllabus but is never used is not the same as one that is worth zero. When the syllabus and the gradebook disagree, ask; do not average them.