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Grade Curve Calculator
Apply a flat, percentage or square-root curve to a raw score.
Teachers & StudentsRaw score 64% (D) curved with the square-root method. Scores are capped at 100%.
When a test turns out harder than intended, teachers often curve the results rather than let one paper distort the term. This calculator applies the three common curve methods to any raw score: adding flat points, boosting by a percentage, or the square-root curve, which lifts low scores the most and top scores the least. Compare the methods on the same raw score to see which one your teacher most likely used, or to choose a fair adjustment for your own class.
How to use the Grade Curve Calculator
- Enter the raw score as a percentage.
- Choose a curve type: flat points, percent boost or square-root.
- Enter the points or boost amount if the method needs one.
- Read the curved score and its letter grade.
Formula
Curved scores are capped at 100%. The square-root curve is the most common formula method in US classrooms: a raw 64 becomes 80, while a raw 90 becomes about 94.9.
Worked example
A raw score of 64% becomes 80% under the square-root curve (10 x 8 = 80), 74% with a flat +10 curve, and 70.4% with a 10% boost (64 x 1.10). A raw score of 90% becomes 94.9% under the square-root curve, 100% with flat +10 after capping, and 99% with a 10% boost. The square-root method deliberately helps the struggling student more.
Good to know
True bell-curve grading, forcing results onto a normal distribution, is different from these score adjustments and is mostly used in large university courses; it can lower as well as raise individual grades. The methods here only raise scores. Whichever method is used, fairness requires applying it to the whole class with the same rule and announcing it, and recording both the raw and curved scores so the adjustment stays transparent at moderation time.
US grading scale reference
The table below is the US plus/minus scale used across this site. Where this tool offers other scales, the Canadian scale sets an A from 85% with a pass at 50%, and the Australian scale uses the bands HD (85%+), D (75-84%), C (65-74%), P (50-64%) and F (below 50%) on a 7.0 GPA.
| Letter grade | Percentage | GPA points |
|---|---|---|
| A+ | 97–100% | 4.0 |
| A | 93–96% | 4.0 |
| A- | 90–92% | 3.7 |
| B+ | 87–89% | 3.3 |
| B | 83–86% | 3.0 |
| B- | 80–82% | 2.7 |
| C+ | 77–79% | 2.3 |
| C | 73–76% | 2.0 |
| C- | 70–72% | 1.7 |
| D+ | 67–69% | 1.3 |
| D | 63–66% | 1.0 |
| D- | 60–62% | 0.7 |
| F | 0–59% | 0.0 |
Frequently asked questions
How does the square-root curve work?
Take the square root of the raw percentage and multiply by 10. A 64 becomes 80, a 49 becomes 70, and a 100 stays 100. It lifts low scores strongly and perfect scores not at all.
What is the fairest way to curve a test?
The square-root curve is widely considered fair because the help is proportional to need. Flat points are simpler to explain. A percentage boost favours already-high scores, so it is used least.
Can a curve lower my grade?
The methods here cannot; they only add. A strict bell curve in a large course can move grades down, which is why many institutions have moved away from it.
Should curved scores be capped at 100%?
Yes, in almost all school policies a curved score cannot exceed 100%. This calculator applies the cap automatically.
How do I curve a whole class at once?
Decide the method and amount from the class distribution, then apply the same rule to each raw score. Running a few representative scores, the highest, median and lowest, through this calculator shows the effect before you commit.
Go deeper
Prefer the full walkthrough? Read our guide: How Does Grading on a Curve Work? — it explains the method step by step with more worked examples.
Sources & further reading
- Education Week — grading and assessment coverage — Reporting and research on classroom grading practice for K-12 educators.
- ASCD — resources on fair and accurate grading — Professional guidance for educators on assessment and reporting.
Related tools
Last reviewed: November 2026. Results from this calculator are estimates based on the published scales above; your school's recorded grade is the official one.