Standard Deviation Explained Without Greek Letters

Standard deviation is the average distance from the mean. To compute: 1) find the mean, 2) subtract the mean from each data point and square the result, 3) average those squares (this is variance), 4) take the square root. In a normal distribution, 68% of values fall within one standard deviation, 95% within two, 99.7% within three (the "empirical rule"). A class with mean grade 75 and SD=5 has tightly clustered students; mean 75 with SD=20 has wild variance. Use our Standard Deviation Calculator for the math.