Short answer. A flat curve adds the same points to every score so the top score becomes 100. A shift curve adds (target average - class average) to every score. A scale curve multiplies every score by target average / class average.
What curving a grade means
A teacher curves a test when the scores are lower than expected. Maybe a question was unclear, or the test was harder than planned. A curve raises the scores by a fixed rule. This calculator shows three common rules so you can see how each one changes the results.
Flat curve
The flat curve finds the top score in the class. It works out how many points that score is below 100. Then it adds that same number of points to every score.
Points added = 100 - top score. New score = old score + points added.
Everyone gets the same boost, so the gaps between students stay the same. The top student ends at exactly 100.
Shift to a target average
Here the teacher picks the average the class should have, such as 80. The tool finds the real class average and adds the difference to every score.
Points added = target average - class average.
This also keeps the gaps the same. It can push the top score above 100. Many teachers then cap scores at 100.
Scale to a target average
The scale curve multiplies every score by the same number instead of adding points.
Factor = target average / class average. New score = old score x factor.
A higher score gets more extra points than a lower one. A 90 gains more than a 50. So this curve rewards students who already did well.
About bell curves
A true bell curve gives fixed shares of the class each letter, for example the top 10% get an A. That needs the whole class list and a set of rules from the teacher. Most teachers who say they curve use one of the three simple methods above. This tool does not rank students into fixed groups.
A note for students
If you do not know all the class scores, you can still test a flat curve. Type your score and the top score you heard about. Schools set their own rules, so check your syllabus or school handbook.
Worked example
Scores: 62, 71, 75, 80, 88. The class average is (62 + 71 + 75 + 80 + 88) / 5 = 376 / 5 = 75.2.
Flat curve. The top score is 88, so add 100 - 88 = 12 points. New scores: 74, 83, 87, 92, 100. New average 87.2.
Shift to 80. Add 80 - 75.2 = 4.8 points. New scores: 66.8, 75.8, 79.8, 84.8, 92.8. New average 80.0.
Scale to 80. Factor = 80 / 75.2 = 1.0638. The 62 becomes 65.96 and the 88 becomes 93.6. New average 80.0.
Questions
What is the fairest way to curve grades?
There is no single fair way. A flat curve and a shift keep every gap the same. A scale curve gives more points to higher scores.
Can a curve lower my grade?
Not with a flat curve or a shift to a higher target. If the target average is below the real average, a shift or scale will lower scores.
Why is my curved score above 100?
A shift or scale can push high scores past 100. Many teachers cap them at 100.
Is this a bell curve?
No. A bell curve gives set shares of the class each letter. This tool uses the three simple curves that most teachers use.
Do I need every score in the class?
For the shift and scale curves, yes, because they use the class average. For a flat curve you only need the top score and your own.
Last checked by the StudyCount team.