Grading Scale Maker
Build a points-to-letter conversion table for any test total.
Each point is worth 2.50 percentage points, so one mark moves a borderline student noticeably.
| Grade | Points | Percentage |
|---|---|---|
| A | 36–40 | 90% and above |
| B | 32–35 | 80% and above |
| C | 28–31 | 70% and above |
| D | 24–27 | 60% and above |
| F | 0–23 | 0% and above |
Cut-offs are rounded up to the next whole mark, so a student needs to actually reach the threshold rather than round into it. On a short test each point is worth a lot — out of 20, a single mark is five percentage points and can cross two grade boundaries.
A grading scale converts raw marks into letters. On a 40-point test using the standard 90/80/70/60 cut-offs, an A starts at 36 points, a B at 32, a C at 28, and a D at 24 — so 23 or below fails.
Build the table once, mark against it
Converting each student's score individually is slow and error-prone. Working out the point thresholds once, before marking, means the whole class can be graded by looking up a number — and it makes the boundaries visible to you in advance, when you can still decide whether they are fair.
Each cut-off is the smallest whole mark that reaches the percentage threshold. On a 40-point test, 90% is exactly 36, so an A starts at 36. On a 35-point test, 90% is 31.5, and since half marks may not exist the threshold rounds up to 32 — a student must genuinely reach the boundary rather than round into it.
Short tests have coarse grades
The most useful thing this table reveals is how much a single mark is worth. Out of 40 points, one mark is 2.5 percentage points. Out of 20, one mark is 5 points — enough to cross an entire grade boundary, and on a plus/minus scale sometimes two.
That is a design problem, not a marking problem. If a single question can move a student from B to A−, the assessment is not fine-grained enough to justify the distinctions the scale is drawing. Either lengthen the test or use fewer grade bands.
Rounded up to the next whole mark, so a student must reach the threshold rather than round into it. One mark is worth 100 ÷ total percentage points.
Worked example: a 40-point test
Convert each percentage boundary into a mark:
- 1 Note the total points. The test is out of 40, so each point is worth 2.5 percentage points.
- 2 Take the A boundary. 90% of 40 = 36 exactly, so an A starts at 36 points.
- 3 Work down the bands. 80% of 40 = 32 for a B, 70% = 28 for a C, 60% = 24 for a D.
- 4 Round up where it is not exact. On a 35-point test, 90% is 31.5, which becomes 32 — the student must reach the boundary, not round into it.
- 5 Set the top of each band. A band runs up to one mark below the next threshold, so a B is 32 to 35 and an A is 36 to 40.
- 6 Sanity-check the granularity. With 2.5 points per mark, one question can shift a borderline student a full grade. On a 20-point test it would be worse.
What one mark is worth
The percentage value of a single point, and how many marks separate adjacent letter grades on the standard 10-point bands.
| Total points | One mark is | Marks per grade band |
|---|---|---|
| 10 | 10.0% | 1 |
| 20 | 5.0% | 2 |
| 25 | 4.0% | 2.5 |
| 40 | 2.5% | 4 |
| 50 | 2.0% | 5 |
| 100 | 1.0% | 10 |
Rounding is a policy, not a calculation
The scale here rounds thresholds up, which is the stricter and more common convention: 89.9% is not an A. Some institutions round the student's percentage instead, so 89.5% becomes 90% and earns the A. Both are defensible, but they are different rules and they must be stated in advance rather than decided when a specific student lands on the boundary.
Whichever you use, apply it uniformly. The borderline case is exactly where a grading scheme is tested, and deciding it case by case is how a scale stops being a scale.
Two further notes. The 90/80/70/60 cut-offs are an American convention and are far from universal — many systems pass at 50%, and some use entirely different band widths, so adjust to whatever your institution mandates. And plus/minus grades on a short test are usually false precision: distinguishing a B+ from a B on a 20-point paper means claiming that a single mark carries real meaning about a student's understanding, which it rarely does.