Rule of 72 calculator
Divide 72 by an annual rate in percent to estimate the years it takes money to double.
How this is calculated
The exact doubling time is ln(2) / ln(1 + r), with r the rate as a decimal. For
small rates ln(1 + r) is close to r, which gives about 69.3 divided by the rate in
percent. The rule uses 72 because it divides evenly by more numbers and is more accurate than 69.3 at typical
rates: it is exact near 7.85%.
The estimate is closest between about 6% and 10%, and drifts further from the exact figure below and above that range.