The Rule of 72 is a ‘rule of thumb’ calculation used to quickly estimate how long it will take for an investment to double in value, given a fixed annual rate of return.
It was first introduced by Italian mathematician Luca Pacioli in 1494, a collaborator of Leonardo Da Vinci. Pacioli is best known as the codification of double entry book-keeping, and the reporting of transactions via journals and ledgers, and outcomes via profit and loss and balance sheet.
His Rule of 72 is widely used in the initial ‘back of the envelope’ assessment of investment options.
The formula is: Years to double = 72/Annual rate of return.
For example, if an investment has an annual rate of return of 8%, it will take around 9 years to double. (72/8 = 9)
The rule can be used to make reasonable estimates of a range of outcomes, such as how long it will take for money to lose value due to inflation, the impact of compounding interest on debt, and evaluating the impact of service fees.
Be careful however, at best the calculations will be estimates, reasonably accurate at rates between 5% and 10%. Outside this range, the accuracy will suffer due to the non-linear nature of compounding growth.
