IV Rank and IV Percentile both look at the current level of implied volatility (IV), but they are completely different metrics.
What is IV Rank?
IV rank compares the current IV to the high & low over the past year
IV rank gauges the current level of IV relative to the IV range over the past 52-weeks.
IV Rank = 100 x (the current IV level - the 52 week IV low) / (the 52 week IV high - 52 week IV low)
What is IV Percentile?
IV Percentile tells you the frequency at current IV levels
IV percentile calculates the percentage of days in the past 52-weeks in which the IV was lower than the current level.
IV% = # days when IV was lower than current IV / 262