Bell Curve Grade Calculator

Grading on a curve assigns letter grades based on how each score compares to the rest of the class rather than to a fixed scale. The scores are mapped onto a normal (bell-shaped) distribution, so a student's grade depends on their rank or their distance from the class mean. This calculator supports two approaches: assigning fixed percentages to each letter (for example 10% A and 20% B), or using standard-deviation cutoffs around the mean.

The percentage method ranks scores from highest to lowest and hands out grades to match target shares, so the top 10% always get an A no matter the raw numbers. The standard-deviation method instead grades by each score's z-score, its distance from the mean in standard deviations, so the actual share of A grades depends on how the scores are spread. Use percentages when you want a guaranteed distribution, and standard deviations when you want to reward genuine separation from the average.

The z-score is (score minus the class mean) divided by the class standard deviation, so it measures how many standard deviations a score sits above or below average. The percentile comes from the standard normal distribution: it is the area under the bell curve to the left of that z-score, expressed from 0 to 100. A z-score of 0 maps to the 50th percentile, while a z-score of about 1.0 maps to roughly the 84th percentile.

The standard-deviation method uses the classic symmetric scheme that centers C on the mean. A is a z-score of 1.5 or higher, B is 0.5 to 1.5, C is -0.5 to 0.5, D is -1.5 to -0.5, and F is below -1.5. Under a perfect normal distribution these bands produce roughly 6.7% A, 24.2% B, 38.3% C, 24.2% D, and 6.7% F.

Yes. In the percentage method the shares for A, B, C, D, and F must total 100% (a small rounding tolerance is allowed). The calculator converts each share into a whole number of students by rounding, then puts any leftover into the lowest grade so the counts always sum to the exact class size.

You can enter between 2 and 1,000 scores, separated by commas, spaces, or new lines. With very small classes a bell curve is statistically rough, so treat the results as a guide rather than a strict rule, and remember that curving can lower some grades as well as raise them.