30-day IV

Also called Constant-maturity IV, 30-day implied volatility, IV30

At-the-money implied volatility at a constant 30 days, blended from the expirations either side of 30 days.

The 30-day IV is at-the-money implied volatility for a constant 30 days to expiration. Real expirations move closer every day, so their IV drifts for reasons that have nothing to do with the market. A constant 30 days makes one day comparable with the next, which is what IV rank and IV percentile need.

How it's built

For each expiration, take the at-the-money IV (as for the expected move). Find the nearest expiration inside 30 days and the nearest beyond it, and blend them in total variance (IV² × time), linearly in time, to exactly 30 days. If every expiration is on one side of 30 days, the one nearest 30 days is used as it is, and the readout says so.

History is one value per session, from that session's closing prices. During the session, today's value follows the latest snapshot until the day's close replaces it.

In Theta Vantage

The first figure on the IV rank panel names the two expirations behind it. The indigo line on the history chart is the 30-day IV, with HV20 in grey beside it.