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What a grading curve calculator does and when to use it
A grade curve calculator raises a class’s test scores with the curve you choose, then shows each curved score and the new class average. By default it adds the same points to every score so the top one becomes 100.
Teachers use it to compare seven curves before posting grades from a hard test. Students use it to check a curve their teacher announced. It works on one test at a time, so carry your curved score into the Grade Calculator to see your class grade.
How to curve grades with the calculator
Type in the scores
Enter each result under Score as a percent (72) or as points (36/50). Student (optional) can hold a name; press + Add student for more rows.
Choose a curve method
Pick a Curve method above the rows; the value it needs (Points or a Target average) sits beside it.
Read the Curved column
Each row shows its new percent and letter. Show an example loads a five-student class.
Grading on a curve: seven formulas and one example
Curved = Score + (100 − Top score)
Score
One student’s raw result on the test, as a percent.
Top score
The highest raw score in the class.
Curved
The new score, never below the raw one and capped at 100.
This is the default flat curve. Take a class of five whose scores are 88, 74, 65, 81 and 57. The top score is 88, so everyone gets 100 − 88 = 12 extra points.
| Student | Score | Math | Curved |
|---|---|---|---|
| 1 | 88 | 88 + 12 | 100% A+ |
| 2 | 74 | 74 + 12 | 86% B |
| 3 | 65 | 65 + 12 | 77% C+ |
| 4 | 81 | 81 + 12 | 93% A |
| 5 | 57 | 57 + 12 | 69% D+ |
The raw scores add up to 365, an average of 73%. The curved ones add up to 425, an average of 85%. The calculator shows 85% B, up from 73% C.
Here are all seven methods on the same five scores, with +5 points, both targets at 80% and a bell curve of 75% with Spread set to SD 10 in Options.
| Curve method | Formula | Curved scores | New average |
|---|---|---|---|
| Add points so the top score is 100 | Score + (100 − Top) | 100, 86, 77, 93, 69 | 85% |
| Scale so the top score is 100 | Score × 100 ÷ Top | 100, 84.1, 73.9, 92, 64.8 | 83% |
| Square root (10 × √score) | 10 × √Score | 93.8, 86, 80.6, 90, 75.5 | 85.2% |
| Move the average to a target | Score + (Target − Average) | 95, 81, 72, 88, 64 | 80% |
| Add the same points to everyone | Score + Points | 93, 79, 70, 86, 62 | 78% |
| Scale the average to a target | Score × Target ÷ Average | 96.4, 81.1, 71.2, 88.8, 62.5 | 80% |
| Bell curve | Average′ + z × SD′ | 88.6, 75.9, 67.8, 82.2, 60.5 | 75% |
How to read your grade after the curve
The big number is the class average once the curve is applied, with its letter. The line under it gives the average before the curve, the number of scores and the method.
Average raised by is the gap between the two averages, in percentage points. Top score and Lowest score show where the best and worst results started and ended, and Passing (60%+) counts scores at 60% or more.
The table under the result counts A’s to F’s before and after and names who lands where. Download CSV is ready for a gradebook.
The letters in the Curved column follow the site’s plus/minus chart by default. Schools set their own cutoffs, so switch Grading scale in Options if your class uses A 90, B 80, C 70, D 60.
What changes a curve, including bell curve grading
One very high score. The flat and scaled curves key off the top score, so one 98 among scores in the 60s adds only 2 points. The square root, target and bell methods avoid this.
The square root curve favors low scores. It lifts a 49 to 70 and a 36 to 60, but turns an 81 into just 90 and a 100 stays 100.
The cap at 100. Curved scores stop at 100% unless you tick Let curved scores go above 100%. A score already above 100 keeps its extra credit.
A curve here never lowers a score, unless you say so. A class already above the target shows “No change”, and a bell curve’s top scores keep their raw score unless you tick Let the curve lower a score.
A true bell curve works differently. In norm-referenced grading, letters depend on where you rank in the class. Johns Hopkins’ Center for Educational Resources describes giving the median score a set grade, such as a C+, and drawing cutoffs by distance from it. This calculator’s Bell curve is the score version: each score keeps its distance from the class average in standard deviations (its z-score). It keeps the class’s own spread unless you set Spread in Options; with SD 10, an 88 in a class averaging 73 (SD 11.05) becomes 75 + 1.36 × 10 = 88.6.
Frequently asked questions
How do you curve grades?
The simplest way is to add the same number of points to every score so the highest one becomes 100. If the top score is 91, everyone gets 9 points. Other methods scale scores by a percentage, use a square root, or move the class average to a target.
How do you curve a test to a class average?
Subtract the current class average from the average you want, then add that difference to every score. A class averaging 68 with a target of 75 gets 7 points each. Pick “Move the average to a target” in the calculator to do it in one step.
What is a bell curve in grading?
A bell curve grades by where each score sits in the class rather than by a fixed percent. This calculator’s Bell curve keeps each student’s distance from the average in standard deviations and moves the class to your target average, with its own spread unless you set one. Ranks stay the same.
Does a grade curve change your GPA?
Only if it changes the letter you get for the class. A curve raises one test score, which nudges your class grade up; your GPA moves only when that class grade crosses into a higher letter.
How do you scale grades so the top score is 100?
Divide 100 by the top score and multiply every score by that number. If the top score is 80, each score is multiplied by 1.25, so a 60 becomes 75. Choose Scale so the top score is 100 in the calculator to do the whole class at once.
Sources
- To Curve or Not to CurveThe Innovative Instructor, Center for Educational Resources, Johns Hopkins University