CBOE SKEW Index¶
OVERVIEW¶
The CBOE SKEW Index (ticker: ^SKEW) measures the perceived tail risk of the S&P 500 over a 30-day horizon by quantifying the price difference between far out-of-the-money (OTM) puts and at-the-money (ATM) options. It is calculated from the same SPX option strip used to compute the VIX, but instead of measuring expected volatility, SKEW measures the asymmetry of that volatility — specifically, how heavily traders are paying up for crash protection.
Value Range
SKEW values range from roughly 100 to 170. A reading of 100 implies a symmetric, normal-distribution outlook. Readings above 130 indicate elevated demand for OTM puts and a fatter left tail.
| SKEW Level | Interpretation | Estimated Probability of 2σ Decline |
|---|---|---|
| 100 | Symmetric outlook, normal distribution | ~2.3% |
| 115 | Mild tail-risk concern | ~5% |
| 130 | Elevated OTM put demand | ~8% |
| 145 | High crash-protection premium | ~12% |
| 155+ | Extreme institutional hedging activity | >12% |
HOW THE SKEW INDEX IS CALCULATED¶
Calculation Formula¶
CBOE derives SKEW from the third moment (skewness) of the implied risk-neutral distribution of 30-day SPX log returns. The formula is:
Where S is the skewness of the 30-day SPX log return distribution implied by option prices. Skewness itself is computed from a strip of OTM SPX puts and calls using a variance-swap-style replication:
Methodology Details¶
The CBOE methodology weights each strike by the inverse square of its price, integrates across the full strike chain, and interpolates between the two nearest expirations to produce a constant 30-day measure. Probabilities of a two-standard-deviation S&P decline rise from roughly 2.3% at SKEW = 100 to roughly 12% at SKEW = 145, based on CBOE's published mapping.
VIX vs. SKEW
VIX measures the level of implied volatility. SKEW measures the shape of the implied distribution. A low VIX combined with a high SKEW — sometimes called the "complacency with hidden hedging" setup — can signal that institutional traders are quietly buying crash protection despite calm surface conditions.
WORKED EXAMPLE¶
On May 14, 2026, the CBOE SKEW Index closed at 142.6 while the VIX closed at 14.20.
The VIX reading suggested calm — implied volatility for ATM SPX options was below the long-run average of 19. But the SKEW reading was at the 88th percentile of its three-year range, meaning traders were paying steep premiums for SPX puts struck 10–15% below spot. This divergence — low VIX, high SKEW — is the classic "complacency with hidden hedging" setup. Eight trading days later, SPY dropped 4.2% over a single week on weaker-than-expected jobs data, validating the tail-risk premium the SKEW had been pricing.
WHEN TRADERS USE THE SKEW INDEX¶
Portfolio managers use SKEW as a sentiment gauge for institutional hedging activity. Persistent readings above 140 indicate that smart money is buying crash protection even when the VIX is low — a warning sign that often precedes corrections. Options traders use SKEW to time put-spread sales: when SKEW is extreme, far-OTM puts are rich relative to closer strikes, making put credit spreads asymmetrically attractive. SKEW is also a key input for risk-parity and tail-hedge funds (e.g. Universa, Cambria) that systematically buy puts when skew is low and sell premium when skew is rich.
LIMITATIONS AND COMMON MISCONCEPTIONS¶
Important Caveats
SKEW is a sentiment gauge, not a crash predictor. Understand these constraints before using it in a strategy.
| Limitation | Detail |
|---|---|
| False positives | Readings above 142 have frequently been followed by quiet markets |
| Missed crashes | Several sharp corrections occurred without an elevated SKEW signal |
| Risk-neutral probability | Published tail-event probabilities overstate real-world crash likelihood due to the volatility-risk premium embedded in OTM puts |
| No timing signal | Elevated SKEW can persist for months without a market event |
| Institutional distortion | Single large hedges can move the index without representing broad sentiment |