Almost every explanation of Canvas’s drop-lowest rule says the same thing: it throws away your worst score. That is close enough to true that nobody questions it, and it is wrong in a way that can cost you a letter grade.
Canvas does not drop your lowest score. It does not drop your lowest percentage either. It drops whichever assignment leaves you with the best possible grade — and once the assignments in a category are worth different numbers of points, those three things stop being the same assignment.
The case that shows it
Say a category holds three graded items, and the instructor drops one:
| Assignment | Score | Percentage |
|---|---|---|
| Pop quiz | 0 / 1 | 0% |
| Midterm | 50 / 100 | 50% |
| Final | 90 / 100 | 90% |
The pop quiz is a zero. It is the lowest score and the lowest percentage, so every explanation you will read says that is the one that goes. Work out what each choice actually leaves you with:
| Drop the pop quiz | (50 + 90) / (100 + 100) | 70.0% |
| Drop the midterm | (0 + 90) / (1 + 100) | 89.1% |
| Drop the final | (0 + 50) / (1 + 100) | 49.5% |
Dropping the zero gives 70%. Dropping the midterm — a 50%, five times the score of the thing everyone assumes gets dropped — gives 89.1%. That is a nineteen point swing, and it is two letter grades.
Why this is not a bug
It would be easy to read that as Canvas behaving strangely. It is the opposite: the rule is there to help you, so Canvas picks the drop that helps most. A rule that mechanically threw away the lowest number would sometimes throw away the one that costs you nothing and leave the one doing the damage.
The formal version, if you want it: Canvas searches for the set of drops that maximises your resulting percentage. With one drop and a handful of assignments you can just try every option, which is what the table above does. With several drops across a long list of homework it is a real optimisation problem, and Canvas solves it properly rather than approximating.
What this means for you
Three practical consequences, in rough order of how often they matter.
A zero on a tiny assignment is usually survivable. Not always — but a one-point participation mark you missed is not the emergency it feels like, and it is frequently not even the thing that gets dropped.
A mediocre score on a heavy assignment is the real problem. A 50% on something worth a hundred points does far more damage than a 0% on something worth one, and it is the one you would want the drop to absorb.
You cannot predict which one gets dropped until everything is graded. The drop is recalculated every time a new score lands. An assignment that is currently being dropped can stop being dropped when the next one comes back, which is why your grade sometimes moves after an assignment you had nothing to do with.
How to check your own courses
In Canvas, open a course, go to Assignments, and look at the assignment group headers. If a group has drop rules, hovering or opening the group settings shows them — normally phrased as “Lowest score is dropped”. Grades themselves live under Grades, where dropped items are usually shown struck through or marked as dropped.
What Canvas will not tell you is what your grade would be without the rule, or which assignment the drop is currently absorbing, or what happens to that when the next score lands.
Related
If you are trying to work out what you need on a final, the free calculator does that in two numbers — though note that a drop rule is exactly one of the cases where the simple version stops being exact.