The classic EZ Grader is a cardboard wheel: dial in the number of questions, and it lines up every possible "number wrong" with its percentage and letter grade. The wheel is just a printed version of one division — and understanding that division explains every grading-chart argument, including the dreaded 89.6%.
The one calculation
Percentage = (Total − Wrong) ÷ Total × 100
Get 3 wrong on a 20-question quiz and you have (20 − 3) ÷ 20 = 85%. Everything else — letters, points, charts — is a lookup from that percentage.
The two common scales
On the standard 10-point scale, 90–100 is an A, 80–89 a B, 70–79 a C, 60–69 a D and below 60 an F. On a plus/minus scale the bands narrow: 93–100 A, 90–92 A−, 87–89 B+, and so on down to D− at 60–62. The same 88% test is a solid B on one scale and a B+ on the other — which is why the scale must be chosen before the grade means anything.
Rounding: the hidden policy
Percentages rarely land on whole numbers. A policy decides what happens at the edges:
- Round to nearest: 89.6% becomes 90% — an A on the standard scale.
- No rounding (exact): 89.6% stays below the 90 cutoff — a B.
- Truncate: 89.9% becomes 89% — also a B.
None of these is cheating or generosity; they are institutional choices. Departments usually set one and apply it to everyone. When you grade with our EZ Grader, set the rounding policy to match your school so the chart you print is the chart your policy produces.
Half points and partial credit
Real marking gives partial credit, so "wrong" is often 2.5 rather than 3. The maths does not care: (20 − 2.5) ÷ 20 = 87.5%. Tools that only accept whole numbers force teachers to round the input before the calculation — an invisible second rounding. Accepting half points keeps the percentage honest.
Why charts differ between websites
Two grading charts disagree when they assume different scales or different rounding — almost never because the division is wrong. Check those two settings first, and the charts will agree.