72法則 - 簡單計算投資加倍所需之年數

72法則在wikipedia的解釋是：
In finance, the rule of 72, the rule of 70 and the rule of 69.3 all refer to a method for estimating an investment's doubling time, or halving time. These rules apply to exponential growth and decay respectively, and are therefore used for compound interest as opposed to simple interest calculations.

公式：72 / (年利率數值) = 投資加倍所需之年數

範例：20歲的小王將50萬存入銀行定存，定存利率2%，何時小王的50萬能增值至100萬呢?
簡單代入公式72 / 2 = 36，經過36年後，小王的50萬才能倍增至100萬，此時小王已是56歲的老灰ㄚ啦～
心得：有閒錢絕不能放銀行定存，若再扣除每年平均3%的通膨，根本別談資產增值，連保值都沒有啦～