Hi
Standard Normal Curve:    μ = 0 and    σ = 1

one  standard deviation from the mean accounts for about 68.2% of the set *****
two standard deviations from the mean account for about 95.4%
and three standard deviations from the mean account for about 99.7%.
Important to Understand z -values as they relate to the Standard Normal curve:

Wish You the Best in your Studies.