What Is Volatility?
Volatility measures the size of an asset's price swings over time. It is typically quantified as the standard deviation of returns, annualized and expressed as a percentage: a stock with 30% volatility moves around far more dramatically than one with 12% volatility.
Importantly, volatility is directionless. It captures how much prices move, not which way, so a stock rocketing higher and one collapsing can both be highly volatile.
Historical vs. Implied Volatility
Historical, or realized, volatility is computed from past price data and tells you how bumpy the ride has actually been. Implied volatility is forward-looking: it is backed out of option prices and reflects how much movement the market expects, which is why option premiums rise ahead of events like earnings announcements.
The VIX index, often called the fear gauge, measures the implied volatility of S&P 500 options over the next 30 days. Readings below roughly 15 suggest calm markets, while spikes above 30 signal fear and stress.
Example
Suppose a stock trades at $100 with an annualized volatility of 25%. Statistically, that implies roughly a two-thirds chance the stock ends the year between $75 and $125, one standard deviation around the starting price. A utility stock at 12% volatility would have a comparable range of just $88 to $112.
Options on the 25%-volatility stock will cost noticeably more than options on the utility, because larger expected swings make the right to buy or sell at a fixed price more valuable.
Why It Matters
Volatility is the standard unit of risk across finance: it drives option pricing, position sizing, portfolio construction, and risk metrics like Value at Risk. Measures like beta compare a stock's volatility to the overall market's.
For traders, volatility is not just a risk to manage but an asset class to trade, and derivatives desks and hedge funds build entire strategies around buying and selling it.
