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The VIX is an index published by Cboe that measures the 30-day volatility of the S&P 500 implied by index option prices. When S&P 500 options grow expensive, the VIX rises; when they cheapen, it falls. It is quoted in annualized percentage points, the same units as any stock's IV.

It earned the nickname "fear gauge" because it tends to spike when stocks fall sharply, but it is more precise to call it the price of index insurance: a reading of what the option market currently charges for 30 days of exposure to S&P 500 movement.

The per-stock IV30 figures on Options Band's volatility pages are conceptually the single-stock cousins of the same idea.

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The VIX's annualized quote converts to a monthly figure the way any IV does: divide by the square root of the number of periods in a year. A worked example with round numbers:

• VIX level: 20
• Horizon: 30 days, roughly 1/12 of a year
• Scaling factor: √12 ≈ 3.5

20% ÷ 3.5 ≈ 5.7%. A VIX of 20 therefore corresponds to an implied one-standard-deviation move of about 5.7% in the S&P 500 over the coming month — roughly a two-in-three chance of finishing within ±5.7%, if the implied distribution were borne out. At a VIX of 40, the same arithmetic gives about ±11.4%.

Historically the index has spent most of its life between the low teens and the low 20s, with excursions above 40 in major crises and readings above 80 at the 2008 and 2020 extremes. Two behaviors recur in the record. It has been strongly mean-reverting: extreme readings in either direction have tended to drift back toward the middle over weeks and months. And it is strongly negatively correlated with the S&P 500 day to day — large VIX jumps overwhelmingly coincide with index declines.

The VIX itself is not tradable. Futures and options on it are, and their prices reflect expectations of where the VIX will stand at their own expirations rather than today's level — a distinction that surprises many first-time readers of VIX-linked products.

詳細説明

The VIX is not the implied volatility of any single option. It is computed from a strip of out-of-the-money SPX calls and puts across strikes, weighted so the portfolio replicates the payoff of a 30-day variance swap; the index is the square root of that implied variance, annualized and multiplied by 100.

VIX² ≈ (2 ÷ T) × Σ ( ΔK ÷ K² ) × e^(rT) × Q(K), summed over OTM strikes K

Because the calculation integrates the whole smile, the VIX loads on tail strikes in a way an at-the-money IV does not — a steepening put skew alone can lift it with no change at the money. Two listed expirations bracketing 30 days are interpolated in variance terms, mirroring the logic of Options Band's IV30 for single stocks.

Misconceptions: the VIX does not measure realized volatility, and its record as a coincident indicator is far stronger than as a leading one — it registers stress as it happens rather than predicting it. Comparing a stock's IV30 with the VIX describes relative pricing of movement, not relative danger. The VVIX (the implied volatility of VIX options) and the VIX futures curve are separate objects answering separate questions.