The Rule of 72 is a simple formula: divide 72 by the annual interest rate to estimate how many years it takes to double your investment.
At 4% → doubles in 18 years | At 6% → doubles in 12 years
At 7% → doubles in 10.3 years | At 8% → doubles in 9 years
At 10% → doubles in 7.2 years | At 12% → doubles in 6 years