Relative class distribution
Bell curve grading calculator
Paste anonymous percentage scores to calculate the class mean, standard deviation, z-score, and default curved letter grade for each paper. No names or score list leave the browser.
Anonymous class distribution
Enter scores from 0 to 100
Separate scores with commas, spaces, or new lines. Use a period for decimals. Up to 200 scores are processed in this tab.
No names, score list, account, or identifier is uploaded or saved.
| Paper | Raw score | Z-score | Letter |
|---|---|---|---|
| Paper 1 | 72 | -1.20 | F |
| Paper 2 | 84 | 0.26 | C |
| Paper 3 | 91 | 1.11 | A |
| Paper 4 | 67 | -1.81 | F |
| Paper 5 | 88 | 0.74 | B |
| Paper 6 | 76 | -0.72 | D |
| Paper 7 | 95 | 1.59 | A |
| Paper 8 | 81 | -0.11 | C |
| Paper 9 | 79 | -0.35 | C |
| Paper 10 | 86 | 0.50 | C |
How this bell curve is calculated
The calculator first finds the arithmetic mean of all valid scores. It then calculates the population standard deviation, which describes how far the scores typically sit from that mean. Each paper receives a z-score using the formula (score − mean) ÷ standard deviation. A positive z-score is above the class mean; a negative z-score is below it.
The default letter bands are transparent: A begins at one standard deviation above the mean, B at one-half standard deviation above, C at one-half below, D at one full standard deviation below, and F below that. These bands describe this calculator’s method, not a universal school rule.
Worked example
Suppose a class mean is 80 and the standard deviation is 10. A score of 90 has a z-score of +1.00 and falls in the default A band. A score of 85 has a z-score of +0.50 and falls in B. A score of 75 has a z-score of −0.50 and falls in C because that boundary is inclusive.
Important limits before grading on a curve
A bell curve ranks performance relative to classmates rather than against fixed learning objectives. A small class or a class with nearly identical scores can produce unstable or unhelpful distinctions. If every score is identical, the standard deviation is zero; GradCal assigns a z-score of zero and the default C band rather than dividing by zero.
Do not paste student names, IDs, email addresses, or other personal information. The tool accepts only numbers from 0 through 100, ignores invalid values, limits a run to 200 scores, and keeps the calculation in the current browser tab. Confirm that curving is permitted by the syllabus, school policy, and any applicable assessment rules before recording results.
When a fixed grading scale is better
Use a fixed scale when every student should be measured against the same thresholds. The grading scale calculator converts a percentage using adjustable A–F cutoffs. Use the test grade calculator for points earned on one assignment, or the weighted and average grade calculator when course categories count differently.
Bell curve questions
Is grading on a curve the same as adding points?
No. Adding the same number of points to every score is a flat adjustment. A bell curve uses the class distribution, so the result depends on the mean and spread of all included scores.
Does this force a certain number of A grades?
No. The default bands are based on z-score thresholds. The number of grades in each band depends on the actual distribution and is not forced to match a fixed quota.
Should absent or missing work be included?
Only if the written policy treats it as a scored zero for this calculation. Mixing missing data with earned zeroes can shift the mean and every curved result, so decide the rule before entering the class set.