IV vs. HV vs. VIX: What's the Difference?

Implied Volatility (IV)

Implied volatility is the volatility figure that, plugged into an options pricing model, produces the option's current market price. It isn't measured directly — it's backed out of what traders are actually willing to pay for an option right now. Because option prices move with supply, demand, and how much uncertainty the market is pricing in, IV rises and falls constantly, and it can differ meaningfully across strikes and expirations for the same underlying (see options skew and volatility term structure).

Historical Volatility (HV)

Historical volatility, sometimes called realized volatility, is a backward-looking statistical measure: the standard deviation of the underlying's own past returns over a chosen lookback window (commonly 10, 20, or 30 trading days), annualized. Unlike IV, HV isn't derived from options at all — it's calculated purely from the underlying's own price history, so it describes what actually happened, not what the market expects to happen next.

The VIX

The VIX (CBOE Volatility Index) is a specific, standardized implementation of implied volatility for the S&P 500. CBOE calculates it by aggregating the weighted prices of a wide range of SPX index puts and calls across two near-term expirations, interpolated to represent a constant 30-day forward-looking horizon. In practice, the VIX functions as the market's single headline number for "how much movement is priced into SPX options right now" — which is why it's nicknamed the market's fear gauge.

How the Three Relate

• IV is the general concept: what any option, on any underlying, is pricing in for the future.
• HV is the reality check: what the underlying actually did, measured after the fact.
• VIX is one specific, standardized number: SPX options' aggregate IV, over a fixed 30-day window, published in real time.

Comparing IV to HV directly is what the IV/HV ratio is for, and comparing an underlying's implied move against its actual historical range is one of the more direct ways traders sanity-check whether options are pricing in a reasonable amount of future movement.

Why This Matters for SPX Options Trading

Every 0DTE and short-dated SPX options decision is, in some sense, a bet on volatility as much as direction — the premium you pay or collect is priced almost entirely off IV. Knowing whether today's IV is elevated or subdued relative to recent HV, and whether the VIX itself is calm or stressed, gives real context for whether options are currently cheap or expensive before you look at a single strike.