# Calculate stock price range using standard deviation

New to stock investing hence reading up a few tutorial to better understand stocks/stock market. Came across a chapter that says, "Understanding risk with the stock market" and it says that traders and analysts use a number of metrics to assess the volatility and relative risk of potential investments (sharpe ratio, sortino ratio, beta, alpha, rsquared), but the most common metric is standard deviation.

I somewhat get what standard deviation is,

• Standard deviation helps determine market volatility or the spread of asset prices from their average price.
• When prices move wildly, standard deviation is high, meaning an investment will be risky.
• Low standard deviation means prices are calm, so investments come with low risk.

There is an example too which says, in a stock with a mean price of \$45 and a standard deviation of \$5, it can be assumed with 95% certainty the next closing price remains between \$35 and \$55. However, price plummets or spikes outside of this range 5% of the time. A stock with high volatility generally has a high standard deviation, while the deviation of a stable blue-chip stock is usually fairly low.

What I did not understand is how did the above example calculate the range of the stock price to be in between \$35 to \$55?

How do you calculate the fluctuation price range if you know the stock price and the standard deviation value? 