Bell curve percentages are various values that are used in the plotting of a density curve to represent a normal distribution in a histogram. The percentages obeyed by all bell curves are 68, 95 and 99.7. The curves are always above the axis.
Each of the percentages in the curve represents a given range of values. All values that are within one standard deviation of the mean are represented by 68 percent, while 95 percent represent those that are within two standard deviations. The 99.7 percent represent values that are within three standard deviations of the center. It is from these values that one can draw the bell curves.