Ask a Professor: How Does Grading on a Curve Work?

What is grading on a curve, and how does it work? A college professor explains how instructors curve grades and what this means for students.

portrait of Genevieve Carlton, Ph.D.
by Genevieve Carlton, Ph.D.

Published April 6, 2022

Edited by Hannah Muniz
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Ask a Professor: How Does Grading on a Curve Work?


After my first college midterm — in inorganic chemistry — the professor announced that the class average was 40%. Students around me groaned. Then, one raised their hand and asked, "Do you grade with a curve?"

Before college, I'd heard of curved grading. It wasn't until that moment, though, that I realized what a huge difference a grading curve might make for my grade.

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"No," the professor answered, "I don't grade with a curve." More groans.

But what is a grading curve exactly? And how does grading on a curve work? The concept of curved grading elicits strong responses from students and professors — and not always for the same reasons.

What Is Grading on a Curve?

Grading on a curve means adjusting student scores after an assignment.

The idea of "the curve" goes back to a bell curve. In a typical class, professors can expect a majority of grades to fall near the class average, with a smaller number of A grades and failing grades. Generally, professors aim for an average grade in the C to B range, depending on the class.

If a professor notices that the average score was significantly lower — for example, the average grade failed to pass — they can grade on a curve. That means modifying each student's grade to raise the average.

In rarer cases, professors might adjust the curve down to lower the average. However, in most cases curved grading will raise the class average.

Curved grading is more common in some subjects than others. Natural science, engineering, and math classes are more likely to use a curve. Fine arts and humanities classes rarely use a curve. And social science classes fall somewhere in the middle.

How Professors Use a Grade Curve

There's not just one way to implement curved grading. Instead, professors take several different approaches.

Professors can move up the average score by a set amount. If the class average was 50%, for example, the professor might raise the average to 75%, effectively giving every student an extra 25 points out of 100.

Similarly, if the average grade was a D, professors might change all D grades to C's. Each student would therefore receive a full letter grade higher than their original grade. Instructors sometimes avoid this approach, however, as it might mean students at the top of the curve earn more than 100% on the assignment.

Another common approach entails taking the highest score on the exam and making that score worth 100%. So if the top-scoring student earned an 85%, the professor would raise every grade by 15%.

This approach doesn't work well, however, if the class has one student who consistently earns high marks. For example, say a student received a 96% but the class average was 60%. In that case, adding 4% won't make much difference.

Professors use other approaches, too. If every student missed the same question on the exam, the professor might drop that question from the final grade calculation to raise the grade. Or they might give students the option to redo questions they missed to partially raise their grades.

Finally, the old standard of extra credit can help raise the class average.

When the Grading Curve Fails

Grading with a curve doesn't always work.

In smaller classes, the grade distributions might be random enough that curving the grades distorts how students perform. Most professors avoid curving grades in classes smaller than around 30-40 students.

Upper-division or graduate-level classes might also not fit the standard grading curve. Seniors taking courses within their major will generally earn higher grades than students new to the subject. Curving these courses can mean lowering the grades of learners who demonstrate mastery of the material — something most professors try to avoid.

Grading curves can also create tension within a class. If you're graded against your peers, you might start to resent the person scoring 98% on the exams for "breaking the curve."

Similarly, if you go into a test assuming only one person in your class will earn an A because of the professor's curve, you may have less incentive to study.

What Grading With a Curve Means for Students

Some professors never grade with a curve. And that can mean shockingly low class averages on exams. Other professors announce their policy at the beginning of the semester, listing how they plan to curve grades in the syllabus.

And some professors only decide to curve grades if their students bomb a test.

As a professor, I find it's sometimes difficult to know whether a particular exam will be easy or challenging for students. If the average is significantly lower than the professor expected, they might apply a curve to only that assignment.

What does grading with a curve mean for students? In general, students like curved grading because it typically means a higher grade.

But curved grading can also feel arbitrary. Rather than receiving a score based on your answers, your grade depends on a mathematical formula applied by your professor.

Should grades reflect your performance compared to others in the class? Or should they reflect your mastery of the material? Even professors disagree on this point.

While I still remember the pain of my chem professor not curving grades, I never used a grading curve as a history professor. Instead, if I noticed that every student struggled with a particular question, I graded their answers more generously.

I also aimed to submit final grades that reflected a student's mastery of the material over the course of the semester rather than on one assignment.

In the end, every professor decides whether to grade with a curve, and students have little power to change their instructor's grading philosophy. Curving grades and not curving grades both underscore the arbitrary nature of grading.


Feature Image: SDI Productions / E+ / Getty Images

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