Realized volatility (HV20)
Also called Historical volatility, HV, HV20, actual volatility
How much the underlying actually moved: the annualized standard deviation of its last 20 daily returns.
Realized volatility measures what the underlying did, not what options price. Theta Vantage's HV20 is the annualized sample standard deviation of the last 20 daily log returns of closing prices:
HV20 = √252 × standard deviation of ln(close ÷ previous close), last 20 sessionsIV − HV
On the IV rank panel, IV − HV is the 30-day IV minus HV20, in volatility points. Positive means options price more movement than the stock has delivered over the last month; that gap is what premium sellers collect, and it's usually positive. A negative gap means the stock has been moving more than options price, which is a poor setup for selling premium.
In Theta Vantage
The grey line on the IV rank history, beside the 30-day IV. The historical move uses a longer window, a year of returns, for strike distances.