5 Common Ways Teachers Grade on a Curve, Explained

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Learn how and why teachers may curve students’ grades
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A grade curve is a relative grading procedure that assigns grades for assignments based on the performance of the class as a whole. There are many reasons a teacher or professor may decide to curve a grade—for instance, if the majority of students performed below what was expected, which may imply that an assignment or test was too difficult. Read on for a guide to common methods of curving grades, their pros and cons, and other ways to help students.

Grading By Curve: Quick Overview

Grading by curve means adjusting individual grades relative to the class’s performance as a whole. Educators often grade on a curve to compensate for lower-than-expected test scores or assignments that were overly difficult. For example, with a standard curve, a teacher may simply add a certain percentage point to each student's grade based on the highest score.

Section 1 of 5:

What is grading on a curve?

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  1. Instructors use a flat or bell curve to adjust individual grades relative to the class’s overall performance. Teachers typically do curved grading when they feel that too many students scored poorly, either because of inadequate instruction or a poorly designed test or assignment. [1]
    • In most cases, a curve will benefit every student by raising their grade somewhat, but in systems like bell curve grading, some student’s curved grade may be lower than their standard letter grade would be.
    • Not every way to curve grades is relative. Curves that involve a minimum F policy only adjust the grades of failing students. Curving using a square-root method means each student’s grade is adjusted differently, and some students benefit more than others.
    • A standard curve (where every student’s grade is curved by the same amount of points) is seen as the most fair because everyone benefits the same amount.
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Section 2 of 5:

5 Common Ways to Curve Grades

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  1. A standard curve sets the highest score achieved as a “100%” and raises all other grades accordingly. To use this method, educators subtract the highest score in the class from the hypothetical "perfect" score. They add the difference to every student’s score, including the highest-scoring one. Using this method, every student gets a better grade after the curve. [2]
    • For example, say the highest grade on a test was 95%. In this case, because 100-95 = 5, a teacher would add 5 percentage points to everyone’s grade. This makes the 95% score an adjusted 100%, and every other score 5 percentage points higher than it was.
    • This method also works using absolute scores, rather than percentages. If the highest grade was a 28/30, for instance, a teacher would add 2 points to the score of every student.
  2. A flat-scale curve boosts every student’s score on an assignment or test by the same number of points. This can be the number of points that an item most of the class missed was worth, or it can be some other (arbitrary) number of points that the teacher thinks is fair. [3]
    • For instance, let's say that the entire class missed one problem which was worth 10 points. In this case, a teacher might choose to add 10 points to every student's score.
    • Because this method doesn't specifically set the highest score in the class as a 100% maximum score, it allows for the possibility that none of the assignments receive a perfect score. It even allows for scores over 100%.
      • An educator can put a cap on the highest possible score to avoid going over 100%, but high-performing students may find this unfair since they are then receiving fewer bonus points than low-performing classmates. [4]
    • To do a flat curve targeting a mean, an educator would add the number of points that’s equal to the difference between the target score and the class average. [5]
  3. A minimum F curve, also known as a minimum F policy, curves low grades by raising them to an arbitrary minimum score. To do this kind of curve, a teacher defines a lower limit for failing grades—a minimum score that is higher than zero, such as 50 (out of 100). Even if students score lower than the minimum on an assignment, it will be the lowest score they can receive, effectively curving their grade. [6]
    • This policy makes it so that very low-scoring assignments have a less drastic effect when averaged with a student's good scores.
    • For example, let's say that a student bombs his first test, scoring a 0. However, since then, he's studied hard, receiving 70% and 80% on his next two tests. Un-curved, he has a 50% grade right now—a failing score. If his teacher sets a lower limit on failing scores of 40%, his new average is 63.3%—a D. It's not a great score, but it's probably fairer than failing a student who's shown real promise.
    • Educators may choose to set separate lower limits for assignments that are turned in vs. assignments that are not. For example, they may decide that for failing assignments, the lowest possible grade is 40%—unless it's not turned in at all, in which case 30% is the lowest possible score.
  4. On a difficult assignment, a class’s average could be surprisingly low. A bell curve grading method shifts grades closer to the teacher’s desired average by fitting grades to a normal distribution. They set the class's mean grade as a C—even if the actual average score was lower than a C. Scores to the left and right sides of the distribution will earn other letter grades, depending on how many points the teacher uses to separate letter grades. [7]
    • To grade on a bell curve, educators determine the class's mean (average) score by adding up all the scores in the class, then dividing by the number of students to find the mean. For example, say they find an average score of 66%.
    • They set this as a mid-range grade. The precise grade is at their discretion—it could be a C, C+, or even B-. Say they set 66% as a C.
    • Next, they decide how many points separate the letter grades in the new bell curve. Generally, bigger point intervals mean that the bell curve is more forgiving to low-scoring students. Let's say they separate grades by 12 points. This means that 66 + 12 = 78 becomes the new B, while 66 - 12 = 54 becomes the new D, etc.
      • They can also calculate the standard deviation and add it to or subtract it from the mean test score to decide where letter grades fall. The mean test score is the lower cutoff for a C grade. One standard deviation above the mean test score is the cutoff for a B. One standard deviation below the mean test score is the cutoff for a D, and so on. [8]
    • Finally, they assign curved grades according to the bell curve system.
  5. A square-root curve method raises low scores significantly without increasing the highest scores by much. This method is time-consuming, as educators have to manually curve each individual’s score. Students may perceive it as unfair because poor performers receive a greater boost on the curve than high-scoring students. However, every score below a 100 will improve. [9]
    • To use a square-root grading curve, a teacher takes the square root of each individual score. They multiply the square root by 10 to get the curved score.
    • For example, say a student’s score is 65. They take the square root of 65 to get 8.06. Next, the teacher multiplies 8.06 by 10 to get 80.6. This is the student’s new score after curving, bringing them from a D to a B-.
    • If a student’s score is 99, a square-root grading curve would shift their score up to a 99.4.
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Section 3 of 5:

Benefits of Grading on a Curve

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  1. Without a curve, a difficult test could tank many students’ grades without accurately reflecting their overall performance in the course. With a curve, educators feel empowered to include more difficult questions knowing not everyone will get a D or an F. It may also be easier to tell who high and low performers are based on the outliers in the curve’s distribution. [10]
  2. If many students score poorly on an assignment, it might have been too difficult for the course’s standards. It could also indicate that the educator failed to effectively teach the material. Either way, the instructor can remedy an unfairly difficult exam or assignment by curving students’ grades—especially if the entire class did poorly. [11]
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Section 4 of 5:

Drawbacks of Grading on a Curve

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  1. Bell curve grading forces students to compete for the highest grades on assignments (or in a class, if final course grades are curved). On a bell curve distribution, even if you get a high score, only the highest scores will earn As and Bs. Your individual performance isn’t enough to earn a certain grade, since the curve makes everyone’s grade relative to the class’s performance as a whole. [12]
    • This competition may cause students to focus on beating each other in the final distribution rather than learning the material. [13]
    • In assignments or courses where most students would not perform well regardless of the curve, the perceived unfairness of the limited amount of high grades is not as much of an issue. The best students should earn As on the curve.
      • Class size is another factor to take into account. A bell curve grading system will likely be more fair when applied to a larger class than a smaller one. [14]
  2. Obviously, when an instructor uses a curve, they’re artificially inflating grades. However, when used sparingly, students’ final grades should still reflect their performance and mastery of the material. Extreme or systematic grade inflation occurs when curving is very frequent and generous, such that low scores earn average grades and average scores earn high grades. [15]
    • Grading on a curve can devalue letter grades as a tool for judging understanding and mastery. Students may have done very well, but not earned an A because other students scored slightly higher than them. Other students may have done very poorly, but earned a passing grade anyway because others did worse than them.
    • For example, if every exam is curved so a D score earns a B, grades for that course may be inflated compared to the same courses run on the standard grading system.
  3. When teachers use a bell curve or other method to adjust students’ grades, everyone may not benefit equally. In some cases, students with high scores will get a lower grade on the curve than they would have on a standard grading system—mostly with bell curve grading, where there are limited numbers of each letter grade to distribute. When this happens, they could feel the grading system is unfair and lose motivation. [16]
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Section 5 of 5:

Other Ways to Help Improve Grades

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  1. Instead of curving, teachers can offer students an opportunity to re-do sections of an assignment they did poorly on. Then, the teacher grades the problems they re-did and offers the students some percentage of the total points they earned on their re-do attempt. They add these points to their first score to get their final grade.
    • Let's say that a student scored 60 points out of 100 on a test. The teacher gives the test back to the student, offering half-credit for any problems she re-does. She re-works the problems she missed, scoring 30 more points. The teacher then gives her 30/2 = 15 more points, making her final score 60 + 15 = 75 points.
  2. Even the best teachers occasionally put unfair or misleading questions on their tests. If, after grading, they find that there are one or two particular items that most of the students seemed to struggle on, they may want to disregard these questions and grade the assignment as if they weren't included.
    • Note that this method gives extra weight to the questions they choose to include. It may also anger students who did well on the questions you chose to eliminate. Teachers can off-set this by turning the tough question into an extra-credit question.
  3. 3
    Assigning Extra-Credit Problems After an assignment that went poorly for some (or all) students, teachers may offer students a special problem, project, or task that, if completed, will raise their scores. This may be an extra problem that requires creative thinking, an extra assignment, or even a presentation. [17]
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Community Q&A

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  • Question
    After watching the video, I'm not sure that is the best way to grade. While that rewards the top student, doesn't it also increase (reward) the grade of the lowest student?
    Community Answer
    The purpose of a curve is to benefit EVERYBODY. It does help the student with the lowest score, but it raises the average and makes you look better. But I see your point! You don't have to use a curve. As said in the article, it's best used sparingly, like when everyone scores badly on the same assignment, because if that happens, the problem might have been with you, not them.
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    Not Helpful 9 Helpful 22
  • Question
    What if the highest grade in my class is already 100 percent?
    Community Answer
    It depends. If, in a class of 40, there was only one student who made 100 percent, and everyone else was under 85 percent, it might me be a good idea to curve the grades. On the other hand, if 10 students out of 40 scored 92 and above, then there's likely no need to curve the grades.
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  • Question
    How do I know if my curved grades are fair?
    Community Answer
    You don’t know because “fair” is subjective. If you feel the curved scores better reflect your situation, then it is “fair.”
    Thanks! We're glad this was helpful.
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      • If you do not want any student to exceed 100 percent when you apply a grading curve, use the class’s highest score as your curve breaker. For example, if more than a 3-point curve will end up putting 1 student's grade over 100 percent, limit your curve to 3 points.
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      References

      1. ↑ https://througheducation.com/how-does-grading-on-a-curve-work/
      2. ↑ https://througheducation.com/how-does-grading-on-a-curve-work/
      3. ↑ https://akhilrao.github.io/blog/teaching/2017/12/22/curving-grades/
      4. ↑ https://througheducation.com/how-does-grading-on-a-curve-work/
      5. ↑ https://akhilrao.github.io/blog/teaching/2017/12/22/curving-grades/
      6. ↑ https://www.ktnv.com/raisingthebar/minimum-f-gives-students-half-credit-regardless-of-work
      7. ↑ https://www.k12academics.com/education-assessment-evaluation/bell-curve-grading
      8. ↑ https://www.sciencing.com/grade-bell-curve-8786404/
      9. ↑ https://www.sciencing.com/grade-using-square-root-curve-8541101/
      1. ↑ https://ii.library.jhu.edu/2013/05/13/to-curve-or-not-to-curve/
      2. ↑ https://apuedge.com/why-grading-on-a-curve-makes-academic-sense-part-ii/
      3. ↑ https://files.eric.ed.gov/fulltext/EJ1256160.pdf
      4. ↑ https://www.sciencedirect.com/science/article/abs/pii/S0272775717308038
      5. ↑ https://blog.nus.edu.sg/provost/2012/01/20/the-bell-curve/
      6. ↑ https://www.ivyscholars.com/what-is-grade-inflation/
      7. ↑ https://ii.library.jhu.edu/2013/05/13/to-curve-or-not-to-curve/
      8. ↑ https://help.d2l.msu.edu/msu-docs/d2l-grades-at-msu/d2l-gradebook-extra-credit

      About This Article

      Article Summary X

      To curve grades, start by finding the highest grade earned in the whole class. Then, subtract that grade from 100. Finally, add that number to every student's grade. For example, if the highest score in the class was 90 percent, you would subtract 90 from 100 and get 10. Then, you'd add 10 percentage points to every student's grade, including the student who scored the highest. To learn other ways you can curve grades, like using a flat-scale curve or a linear-scale curve, keep reading!

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