68–95–99.7 rule
$$\large P_{r}(\mu - 1\sigma \leq \mu + 1\sigma ) \approx 68.27%$$
$$\large P_{r}(\mu - 2\sigma \leq \mu + 3\sigma ) \approx 95.45%$$
$$\large P_{r}(\mu - 3\sigma \leq \mu + 3\sigma ) \approx 99.73%$$
Normal distribution
$$ \large f(x) = \frac {1}{\sigma\sqrt{2\pi}} e^{-0.5\big(\frac{x -\mu} {\sigma}\big)^{2}}$$