How does Grading on a Bell Curve Work​?

Let’s get straight to the point: A grade curve is a relative grading method that assigns grades based on the class’s overall performance, which often leads students to ask, “But, how does grading on a bell curve work?” 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.

If you’re also curious about the potential downsides of curved grading, you may want to read our related article: “Can Grading on a Curve Hurt You? What Students Should Know”.

For now, in this article, we will answer the following questions:

• What is a grading curve?
• How does grading on a curve work?
• What does grading on a curve mean?

First, you may wonder: “What does grading on a curve mean in practice?” Essentially, as we said earlier, it means adjusting individual grades in relation to the class’s overall performance. For example, to illustrate the question “How does curve grading work?” at a basic level, a teacher may add a certain percentage point to each student’s grade based on the highest score.

Teachers typically apply grading on a curve when they feel that too many students scored poorly. In most cases, a grading curve will benefit every student by raising their grade somewhat; however, in systems like bell curve grading, some students’ grades may be lower than they would otherwise be.

If you’re interested in the math behind these adjustments, our guide “How to Grade on a Curve: Formulas and Methods” can help you walk through the calculations in more detail.

To really understand the question “How does a bell curve work in grading?”, it is helpful to examine how grades tend to spread out. For example, on a difficult assignment, the class average might be surprisingly low. And that’s when the bell curve grading method comes into place; basically, it brings grades closer to the teacher’s desired average by adjusting grades to a normal distribution.

Some teachers take it a step further and calculate the standard deviation, then add or subtract it from the average test score to determine the grade thresholds. The average score of the test can serve as a threshold for a C, while one standard deviation above or below defines higher or lower grades. This statistical approach further explains how grading on a curve works in bell curve systems.

For a broader discussion, you may want to check our article: “What Does It Mean to Grade on a Curve? The Philosophy of Relative Assessment”.

Here are the most common ways professors apply a grading curve in practice.

Flat-Scale Curve

A flat-scale curve boosts every student’s score by the same number of points. This approach illustrates how grading curves work when instructors aim to compensate for a specific question that most students have answered incorrectly.

Standard curve

A standard curve sets the highest score achieved as 100% and increases all other scores accordingly. Using this method, teachers subtract the highest score from a perfect score and add the difference to each student’s grade. This approach is one of the clearest examples of how grading curves work.

Minimum F curve

A minimum F curve increases very low grades to a predefined minimum score. This method mitigates the impact of extremely low scores and demonstrates another variation in how grading on a curve works, without negatively affecting high-performing students.

Square-root curve

A square root grading curve significantly increases low scores without substantially increasing high scores. To use a square root grading curve, a teacher takes the square root of each individual score, then multiplies the square root by 10 to get the curved grade. Although each score below 100 improves, students often perceive this method as uneven, which affects how the notation on the curve is viewed. For example, let’s say a student’s score is 65. They take the square root of 65 to get 8.06. Then the teacher multiplies 8.06 by 10 to get 80.6. This result adjusts their grade from a D to a B-.

Grading on a curve has many benefits:

• One of the advantages of curve grading is that teachers can evaluate students on more difficult material without unfairly penalizing the entire class.

• Grading on a curve can also compensate for assignments that were too difficult or poorly designed. If you want to go deeper into the subject, check out the following article: ”Does Grading on a Curve Help You? What Professors Don’t Always Explain”.

However, curve grading also has disadvantages:

• It can put students in competition with each other, especially under bell-curve grading. This competition can lead students to focus on their peers’ performance rather than learning the material.

• Grading on a curve can also contribute to the inflation of the grade if it is used too frequently or too generously.

• Finally, students may think that the grading on curve method is unfair when good performances get lower grades due to relative ranking.

So, how does grading on a bell curve work for students? Well, it depends on how the grading curve is applied and on the teacher’s transparency about the process. Therefore, understanding how curve grading and, most importantly, how a bell curve works in grading does not give students control over grading policies. Still, it does bring clarity to what often seems opaque.