How to Curve Grades: Four Common Methods With Examples

Why teachers curve

A curve changes raw scores after a test, usually because the test was harder than intended or a question was unclear. Whether to curve, and how, is up to the teacher and school policy. These are four common methods with the arithmetic for each.

Add a flat number of points

Add the same number of points to every score. With a 5-point curve, 72 becomes 77 and 91 becomes 96. Cap results at 100 if your policy requires it.

This is simple to explain, and every student gains the same amount. It does not change the gap between students, and it helps low scores proportionally more than high ones.

Scale the top score to 100

Divide each score by the highest score and multiply by 100. If the top score is 92, a 74 becomes 74 ÷ 92 × 100 = 80.4, and a 46 becomes 50.

This rewards everyone in proportion, but one unusually high or low top score moves every other grade. It also assumes the top student earned a full score on a fair test.

The square root curve

Take the square root of the score and multiply by 10. A 64 becomes 10 × √64 = 80, a 49 becomes 70, and an 81 becomes 90. Low scores gain the most, and a 100 stays 100.

It is quick on a calculator, but students may find it harder to follow than a flat bonus.

Set a target average

Choose the class average you want and add the difference to every score. If the class average is 68 and the goal is 75, add 7 points to each score. This is a flat curve with the amount chosen from the data.

Fairness notes

Tell students which method you use and apply it to everyone. A curve also moves letter cutoffs: a 70 after the curve may earn a different letter than a 70 before it, so state whether cutoffs apply to raw or curved scores. Keep the raw scores in the gradebook so you can change the method later.