Although the VIX Index formula is complex, there are some basic points to understand about how the VIX is calculated.

The VIX Index is calculated in **real-time** using the **midpoint **of **S&P 500® Index (SPX) Call and Put option bid/ask quotes** that are listed for trading on the Cboe Options Exchange.

Only **standard SPX options** and **weekly SPX options** that **expires on Fridays** are used for the calculation.

- Standard SPX options expire on the third Friday of each month
- Weekly SPX options expire on all other Fridays.

The options used for the calculation are **out-of-the-money** SPX calls and puts centered around an at-the-money strike price and with **more than 23 days and less than 37 days to expiration (DTE)**. These options are then weighted average to yield a constant maturity **30-day** measure of the expected volatility of the S&P 500 Index.

The generalized formula used in the VIX Index calculation is:

The square root of that calculated value is multiplied by 100 to get the VIX.