Turn a total return, earned over any period, into its per-year (yearly) equivalent.
Takes the nth root of the total growth multiple, where n is the number of years.
CAGR: (End ÷ Start)^(1/years) − 1
Why not just divide the total return by the number of years?
Simple division does not account for compounding. A 100% total gain over 2 years is not a 50%-per-year return — it is about 41.4% per year, because each year's gain builds on the growth from the year before.