What is the probability that a randomly selected, normally distributed value lies below one standard deviation from the mean?

In a normal distribution, the mean divides the bell-shaped curve into 2 parts: each one = 50% or 0.5

The probability within - one σ and + one σ = 68%

and from the mean to - one σ = 34%

The left par of the mean = 50% and from the mean to - one σ = 34%, then

the probability that lies below one standard deviation from the mean is:

50% - 34% = 16% or 0.16