The square root grading curve is a method for raising the grades of an entire class to bring them into closer alignment with expectations. It can be used to correct for unexpectedly difficult tests or as a general rule for difficult classes. It adds more points to lower scores, but will not result in any scores above 100 or in lower raw scores being curved to exceed higher raw scores.
Take The Square Root of the Raw Score
Take the square root of the raw score. Round the result to one decimal place beyond the scores recorded in your grade book. For example, if you typically grade to one decimal place, a raw score of 88 would result in the square root 9.38.
Multiple by 10
Multiply the square root of the raw score by 10 to get the curved score. In the example above, the final score would be 93.8.
Repeat for all grades in the class.
TL;DR (Too Long; Didn't Read)
The square root curve can be applied more than once if necessary. Low scores will continue to be more greatly affected than high scores, and high scores will not exceed 100 percent.
The square root curve may not work as intended if you are not using a 100 point grading system. Grades should always be expressed as a percentage value when using this curve.