Volatility Skew: Why Implied Volatility Differs by Strike

Educational content about U.S.-listed equity, ETF, and broad-based index options only; not individualized investment, legal, or tax advice. Options involve risk and may lose the entire premium or more, depending on the position.

Direct answer

Volatility skew is the pattern of different implied volatilities across strikes or Deltas for options on the same underlying and expiration. In U.S. equity and broad-based index options, downside OTM Puts often have higher IV than ATM options or comparable upside Calls. The relative premium can reflect protection demand, negative spot-volatility dependence, jump and correlation risk, dealer inventory, capital constraints, and liquidity.

Skew is a relative price shape, not a direct probability forecast. A rich downside Put does not prove an immediate decline is expected, and it does not make selling that Put automatically attractive. Its premium may compensate for losses that arrive when the underlying, volatility, correlation, funding, margin, and execution all deteriorate together.

Scope and limits

This page covers exchange-listed options on U.S. equities, ETFs, and broad-based indexes and was fact-checked on 2026-08-23. Contract multiplier, deliverable, exercise style, settlement, trading hours, margin, tax treatment, and account permissions vary by product, corporate action, broker, account type, and jurisdiction. Confirm current contract specifications, the OCC disclosure document, broker terms, and applicable local rules before trading.

The figures below are hypothetical, not live market data, historical estimates, or forecasts. IV and Delta are model-dependent values inferred from option prices and inputs. Results can change with the pricing model, rates, dividends, borrow, forward estimate, timestamp, quote quality, and Delta convention. Displayed midpoint or fitted values may not be executable. Nothing here determines suitability or provides individualized investment, legal, or tax advice.

How to measure skew consistently

Fix the underlying, quote timestamp, and expiration first. Then choose and disclose one coordinate:

• Delta skew: compare matched-absolute-Delta OTM Put and Call IV, such as 25Δ Put IV − 25Δ Call IV.
• ATM-to-wing skew: compare a tail option’s IV with a documented ATM definition.
• Moneyness slope: compare IV against K/F, log-forward moneyness ln(K/F), or another fixed range.
• Risk-reversal quote: often Call IV minus Put IV, although some markets or vendors reverse the sign.

These measures are not interchangeable. Delta depends on IV, time, rates, dividends, forward, and convention. A “25 Delta” can mean spot or forward Delta and may be premium-adjusted. A fixed strike also changes economic moneyness after spot or forward moves. Record the full definition before comparing dates, assets, or vendors.

Do not treat raw Put-minus-Call premiums as IV skew. Intrinsic value, strike, forward, rates, dividends, and time affect dollar prices. IV maps price through a model onto a more comparable scale, but remains model- and input-dependent.

Skew is one slice of a volatility surface. It differs by expiration and can contain event-specific kinks. It can steepen, flatten, rotate, or move with the overall IV level. A two-point slope cannot describe wing curvature or term structure. The capitalized Cboe SKEW Index is a specified SPX-based benchmark, not a generic synonym for a 25Δ risk reversal or every product’s skew.

Hypothetical downside-skew reading

For one expiration, suppose synchronized, executable quotes processed with one model imply:

Under a Put-minus-Call convention:

25Δ skew = 32% − 24% = +8 volatility points

Under a Call-minus-Put risk-reversal convention, the same quotes produce 24% − 32% = −8 points. Nothing economic changed; only the sign convention did. The 10Δ Put − ATM difference is 40% − 26% = 14 points, showing that the far downside wing is richer than one 25Δ slope reveals.

Selling the 25Δ Put because its IV is 8 points above the Call does not lock in an 8-point gain. A selloff can move the Put toward ATM, lift the entire IV level, steepen skew, widen Bid/Ask, and increase margin simultaneously. Buying the rich Put pays for protection but can still lose the premium if the feared move does not occur before time value decays.

Analysis and risk controls

• Use synchronized, executable Bid/Ask quotes; reject stale, crossed, zero-bid, or anomalously wide wing quotes.
• Estimate forward, dividends, rates, borrow, and time consistently before calculating moneyness, Delta, and IV.
• Compare the same expiration and coordinate; label the Delta convention, ATM rule, sign, and interpolation.
• Plot more than 2 points. A risk reversal measures one relative difference, not the full smile or smirk.
• Inspect each expiration; earnings, macro events, distributions, or index rebalancing can create local term and strike features.
• Stress spot down and up jointly with IV level, skew rotation, term structure, elapsed time, and spread widening.
• Revalue every leg. Net Vega near 0 can leave substantial skew, Vanna, Gamma, and basis exposures.
• Size short-wing positions from gap, liquidity, and margin stress, not premium received or historical win rate.
• Include early assignment, expiration, settlement, adjusted deliverables, and the account’s ability to fund or carry the result.
• Attribute P&L to spot, time, IV level, skew and curvature, model inputs, and execution so directional loss is not mislabeled as skew.

Common misconceptions

• “Puts are always more expensive than Calls.” Dollar prices are not skew; relative IV depends on expiration and coordinate.
• “Steep downside skew predicts a crash.” Skew combines risk premiums, demand, inventory, constraints, liquidity, and beliefs, not one physical probability.
• “High Put IV makes selling favorable.” Rich compensation can accompany severe negative-Gamma, Vega, gap, assignment, and margin risk.
• “Skew is one universal number.” Sign, Delta, ATM, strike range, expiration, and fitting method differ.
• “A 25Δ measure describes the whole surface.” It misses farther wings, curvature, events, and term structure.
• “Fixed-strike comparisons are stable through time.” Spot and forward moves change moneyness.
• “Net Vega of 0 removes skew risk.” Different strikes can reprice in opposite relative directions.
• “A smooth fitted skew is tradable.” Midpoints and interpolated wings may lie outside executable markets.

Related topics

• Skew Dynamics
• Risk Reversal
• IV Crush vs. Skew
• Implied Volatility

Authoritative sources

• April Webinar Key Takeaways: Understanding Volatility and Options Skew — Options Industry Council
• Dawn of a New Era Brings on the Existence of Skew — Cboe Global Markets
• Implied Binomial Trees — The Journal of Finance
• Characteristics and Risks of Standardized Options — Options Clearing Corporation