How to read it
IV rank
Where the reading sits between the lowest and the highest in the window: the reading minus the low, divided by the high minus the low. One spike sets the high and keeps the rank low for months.
IV percentile
The share of days in the window with a lower reading than the one picked. It counts days, so one spike does not stretch it.
Why they differ
After a spike, rank stays low while percentile is high: most days were below the reading, but the range is wide. The buttons jump to the date where the two are furthest apart in the window you picked.
Windows and series
The window is in sessions, not calendar days: 252 is about a year. The Nifty series is computed by RupeeCase from NSE settlement prices; India VIX is the value NSE publishes. Both definitions are taught in the IV rank and percentile module.
What it leaves out
A window is a choice and the answer moves with it. A high rank says implied volatility is high against its own recent range, not that options are expensive: realised volatility is the other half of that comparison. The file holds 300 sessions from 1 Jul 2025, so a 252 session window exists only for the last 49 dates, and a shorter window opens up earlier dates.