The BitMEX Daily Historical Bitcoin Volatility Index is referred to as the .BVOL24H Index. This index is calculated logarithmic percentage change taken from measurements taken the Bitcoin spot price every minute.
The settlement price is calculated from 1440 snaps over the 24-hour period.
P = Last Price (taken at 1 minute intervals)
Stdev = Sample Standard Deviation
Ln = Natural Logarithm
Sqrt = Square Root
.BVOL24H Index = Stdev(Ln(P1/P0), Ln(P2/P1), ..., Ln(P1440/P1439))