What is Volatility Skew? Definition, Formula, and Example
Volatility skew is the pattern where options at different strike prices on the same underlying carry different implied volatilities, with equity put options consistently priced at higher IV than equivalent calls due to persistent demand for downside protection.
What is Volatility Skew?
Volatility skew describes the pattern where options contracts on the same underlying with the same expiration but different strike prices carry different implied volatilities. In equity markets, put options struck below the current price consistently carry higher implied volatility than calls with equivalent distance above — a configuration called "negative skew" or "put skew." The pattern is not random; it reflects the market's chronic demand for downside protection and the empirical reality that equity crashes are sudden while rallies are gradual. Vol skew is one of the most information-dense signals in the options market.
How Volatility Skew Is Measured
Skew is quantified by comparing implied volatility at different option strikes or delta levels:
25-delta Risk Reversal = IV(25-delta call) − IV(25-delta put)
When negative, the market is "put-skewed" — puts cost more than equivalent calls. A reading of −5 means 25-delta puts carry 5 implied vol points more than 25-delta calls.
Skew Ratio = IV(OTM put) / IV(ATM option)
Values above 1.0 indicate a put skew. Term structure of skew refers to how this ratio changes across expirations — short-dated skew is typically steeper around binary events (earnings, FOMC).
Worked Example
With SPY trading at $520 in early 2025:
| Strike | Type | IV |
|---|---|---|
| $546 (+5%) | Call | 12% |
| $520 (ATM) | Call/Put | 14% |
| $494 (−5%) | Put | 19% |
The 25-delta risk reversal is approximately −5 vol points. A trader selling the $494 put collects significantly more premium than they would selling the equivalent-distance $546 call. This difference is not mispricing — it is the risk premium the market charges for tail exposure.
When Traders Use Volatility Skew
Options traders monitor skew to compare the current fear premium against historical norms, identifying whether downside protection is cheap or expensive relative to its own history. A steep put skew signals institutional hedging demand and often precedes broad distribution. Volatility arbitrageurs trade "skew" by taking opposing positions across strikes and delta-hedging the underlying, profiting when skew normalizes. Risk reversal traders sell expensive puts and buy cheap calls when skew reaches extreme levels, targeting a return to the mean. Skew also determines the relative attractiveness of credit spreads: wide skew makes put spreads richer than call spreads for the same width.
Limitations and Misconceptions
Skew does not predict the direction of the underlying — it reflects the cost of hedging, not certainty of a move. Elevated put skew can persist for months in calm markets. Skew can invert in melt-up environments where call demand dominates, as occurred during certain meme-stock episodes in 2021. Comparing raw IV numbers across strikes ignores delta, which changes as the underlying moves. Skew also varies by sector: technology stocks show steeper skew than utilities, and single-name skew behaves differently from index skew because index puts hedge entire portfolios, commanding a structural premium.