What is the VIX? Definition, Formula, and Example
The VIX is the CBOE Volatility Index, a real-time measure of the market's 30-day implied volatility expectation for the S&P 500, derived from SPX option prices and expressed as an annualized percentage.
What is the VIX?
The VIX is the CBOE Volatility Index — the market's real-time estimate of how much the S&P 500 will move over the next 30 calendar days, expressed as an annualized percentage. It is constructed from the prices of S&P 500 index options (SPX) across a wide range of strikes and two expiration dates, using a model-free methodology. A VIX reading of 20 means the options market is pricing in ±20% annualized volatility, which converts to approximately ±5.77% per month and ±1.26% per day.
How the VIX is Calculated
The CBOE's VIX methodology interpolates between the two nearest monthly SPX option expirations to produce a constant 30-day implied volatility. Rather than relying on a single at-the-money strike (as the original 1993 VIX did), the current version aggregates pricing information across the full options chain:
VIX = 100 × √[(2/T) × Σ(ΔKᵢ/Kᵢ²) × eʳᵀ × Q(Kᵢ) − (1/T)(F/K₀ − 1)²]Where T = time to expiration, F = forward price of the S&P 500, K₀ = the first strike below F, Kᵢ = each strike in the chain, and Q(Kᵢ) = the midpoint of the bid/ask for the option at that strike. The result is interpolated between the two expiration dates to lock in exactly 30 days.
Quick conversion:
Expected daily move (1σ) = VIX / √252
Expected weekly move (1σ) = VIX / √52Worked Example: COVID Spike vs. Calm 2017
On March 16, 2020, the VIX closed at 82.69 — surpassing its 2008 financial crisis peak. That reading implied a daily 1σ move of 82.69 / √252 = 5.21% for SPY. Actual SPY moves that week were −11.98%, −4.94%, and −5.18% — the market realized even more volatility than implied at the extremes. By contrast, throughout 2017 the VIX averaged 11.1, implying daily moves of just 0.70%. SPY's actual realized volatility that year averaged 6.7% annualized — options were expensive even at those low VIX levels.
When Traders Use the VIX
Options traders use VIX level and direction to calibrate premium. When VIX is elevated, options are expensive — premium sellers (short straddles, iron condors) benefit from the subsequent IV crush. When VIX is depressed, options are cheap — premium buyers and long vol strategies benefit from any volatility expansion. Equity traders use VIX as a contrarian sentiment gauge: VIX readings above 30 have historically co-occurred with equity market bottoms as fear peaks. Macro hedgers use VIX futures or VIX call options to protect equity portfolios during dislocations.
Limitations and Misconceptions
VIX measures implied volatility — the market's expectation — not realized volatility. Implied consistently overstates realized about 70% of the time, which is why systematic options selling has a structural edge. VIX is also not a directional indicator: a rising VIX does not mechanically mean the S&P falls, though the two are strongly correlated. VIX ETPs like UVXY and VIXY suffer severe daily decay from rolling short-dated VIX futures and do not track spot VIX over multi-day holds. Intraday VIX spikes can be brief and fully reverse by the close.