In a bell-shaped standard normal curve, mean +-2 standard detions covers:

If the distribution of observations is bell-shaped, then even more can be said about the percentage of observations that lay between the mean and +- 2 standard detions. For a bell-shaped distribution, approximately: 67% of the observations lie between the mean +- 1 standard detion 95% of the observations lie between the mean +- 2 standard detions 99.7% of the observations lie between the mean +- 3 standard detions Ref: Dawson B., Trapp R.G. (2004). Chapter 3. Summarizing Data & Presenting Data in Tables & Graphs. In B. Dawson, R.G. Trapp (Eds), Basic & Clinical Biostatistics, 4e.