Rule of 72 was created some 100 years ago, when there were no calculators and computers to estimate to doubling time of an investment. But today, it is considered obsolete. If you took a rate such as 10%, how long would it double the investment? You would simply divide 72/10=7.2 years for the investment to double. But with modern calculators and computers, you can get the exact period of time that the investment would double at 10%. A calculator will give you: 7.27 years for the investment to double at 10%. A t 15%, it would take 72/15 =4.8 years, while with a calculator, it would be: 4.96 years exactly and so on.....
The formula you would use for the calculator would be this: FV =PV[ 1 +R]^N =2=1[1 +R]^N. If you know R=interest rate, then you would solve for N=number of years to double the investment.
