Historical Volatility
Historical volatility (HV) is the annualized standard deviation of a stock's daily price returns over a defined lookback period, measuring how much the stock has actually moved in the past.
Historical volatility is a backward-looking measurement of price variability, in contrast to implied volatility, which is forward-looking. It is calculated by taking the log returns of daily closing prices over a chosen window — commonly 10, 20, 30, or 60 trading days — computing their standard deviation, and annualizing the result by multiplying by the square root of 252 (the approximate number of trading days per year in U.S. markets).
Historical volatility serves as a benchmark for evaluating whether current implied volatility is relatively expensive or cheap. If a stock's 30-day historical volatility is 18% but its options are trading with an implied volatility of 30%, options appear overpriced relative to realized movement. Premium sellers often seek this scenario — selling options with elevated IV while expecting the stock's actual movement to remain closer to its historical volatility, allowing the inflated premium to decay profitably.
Conversely, when implied volatility is lower than historical volatility — a less common situation — options appear underpriced. Traders may purchase options anticipating that the stock will continue to move as actively as it has in the recent past, but the market is not yet pricing that activity into premiums.
The difference between implied volatility and realized (historical) volatility is called the 'volatility risk premium' (VRP). Academic research and industry data consistently show that implied volatility tends to exceed realized volatility on average in equity markets. This VRP is the theoretical underpinning for why premium-selling strategies have historically been profitable over time — investors are willing to overpay for options as insurance, creating a structural edge for disciplined sellers.
Historical volatility is not static. Stocks in low-news environments may have 20-day HV near 15%, while the same stocks during earnings season or a market correction may have 20-day HV above 50%. Traders adjust their strategy selection based on the HV regime — avoiding short-premium strategies in high realized volatility environments where gap risks are elevated, and favoring them in stable, low-HV regimes.