Grading Bell Curve Calculator
Enter your score, the class average and the standard deviation to see your z-score, percentile and the letter grade a standard bell curve would give.
A true bell curve grades you against the class rather than against a fixed percentage. Your standing is measured by the z-score — how many standard deviations above or below the class average you scored — and grades are assigned in bands around the mean.
How the bell curve grade is worked out
First find your z-score, then read off the grade band. The percentile is the share of the class expected to score below you.
Z-score = (Your score − Class mean) ÷ Standard deviation
| Z-score range | Grade | Rough share of class |
|---|---|---|
| Above +1.5 | A | ~7% |
| +0.5 to +1.5 | B | ~24% |
| −0.5 to +0.5 | C | ~38% |
| −1.5 to −0.5 | D | ~24% |
| Below −1.5 | F | ~7% |
Worked example
You scored 85; the class mean was 70 with a standard deviation of 10:
| Z-score = (85 − 70) ÷ 10 | +1.5 |
| Grade band | B (top of the B range) |
| Percentile | ≈ 93rd |
Frequently asked questions
What is a z-score?
It's how many standard deviations your score is above (positive) or below (negative) the class average. A z-score of 0 is exactly average.
Do all teachers use these exact bands?
No. The ±0.5 and ±1.5 bands are the classic textbook cut-offs, but instructors set their own. Use this as a guide, not a guarantee.
Where do I get the mean and standard deviation?
Your instructor may share them, or you can calculate them from the full list of class scores.
Related calculators
An estimate using the classic bell-curve bands and a normal distribution. Real grading policies vary by instructor.