Weighted grading sounds like a simple multiplication problem, and it is. The trouble comes from the edge cases: empty categories, extra credit, and gradebooks that quietly calculate things differently from the way the syllabus describes them.
The basic formula
Each category gets a percentage of the final grade. You work out the student's percentage within each category, multiply by the weight, and add them up.
Say your syllabus uses:
| Category | Weight | Student's category average |
|---|---|---|
| Tests | 40% | 78% |
| Assignments | 30% | 92% |
| Projects | 30% | 85% |
The final grade is 0.40 × 78 + 0.30 × 92 + 0.30 × 85 = 31.2 + 27.6 + 25.5 = 84.3%.
Notice what that means in practice. One more 90 on a test moves the grade far more than one more 90 on an assignment, because each test is competing for a share of a bigger slice.
Category average: points or equal weight?
Inside a category, there are two ways to compute the average, and they don't give the same answer.
- Total points: add up all points earned and divide by all points possible. A 100-point test counts five times as much as a 20-point quiz.
- Equal weight: each item counts the same regardless of its point value. The 20-point quiz counts as much as the 100-point test.
Many gradebooks default to total points within a category. If your syllabus implies every quiz counts equally but the quizzes have different point values, students will see grades they can't reproduce by hand.
The empty-category problem
In week two, you might have assignments in the book but no tests. Most systems handle this by dropping the empty category and re-scaling the others. With only assignments (30%) and projects (30%) present, each now effectively counts 50%.
That's reasonable, but it causes a predictable surprise: the first test lands and a student's grade drops eight points overnight. Telling students and parents about this up front saves a lot of emails.
Extra credit in a weighted system
Extra credit points added to one category only affect that category's share. Ten bonus points in a 10% homework category barely move the final grade; the same ten points in the test category move it a lot. Decide which category bonus work belongs to, and say so. Our post on extra credit covers whether to offer it at all.
Checking your gradebook
Pick one student each grading period and calculate their grade by hand from the raw scores. If your number and the gradebook's disagree, the usual causes are:
- A category with the wrong weight, or weights that don't add to 100.
- Points-based averaging where you expected equal weighting.
- An excused assignment entered as a zero (or the reverse).
- A "missing" flag that the system treats as zero.
If you grade with rubrics, the step from rubric scores to a percentage is its own source of confusion; see converting rubric scores to grades. And if students keep asking what they need on the final, point them to how to calculate the grade you need on a final.