APY = ( 1 + APR / n ) ^ n – 1
Periodic Rate = ( 1 + APY ) ^ ( 1 / n ) – 1
n = Frequency of compounding within a year
APY is used in the context of net returns expected by investors: i.e. equity return rate.
For any given APR, the corresponding APY will always be a little higher depending on how many times it is compounded within the year.