Therefore, we can see that the mean +/- SD contains 66.7% of the area under the curve. i.e. total area of + 1SD and -1SD  is equal to 66.7% (33.35% +33.35%). Similarly, the mean +/- 2 SD contains 95% of the area under the normal curve and the mean +/- 3 standard deviations contains 99.7% of the area under the normal curve.