Estimate how many years it takes an investment to double at a given annual growth rate.
How It Works
How Rule of 72 Calculator Works
Dividing 72 by the annual percentage rate gives a quick mental-math estimate of the doubling time — it works because of how compound growth curves behave, and it stays reasonably accurate for typical rates between about 4% and 15%. Enter a target number of years instead of a rate, and the calculator solves it the other way around, telling you the growth rate needed to double your money by then.
FAQ
Frequently Asked Questions
Why 72 specifically?
It falls out of the math behind compound growth (the natural logarithm of 2, scaled to percentage terms) and happens to divide evenly by more small numbers than the more mathematically exact 69.3, which is why 72 became the memorable rule of thumb.
How accurate is this compared to the exact compound interest formula?
Very close for typical rates — within a few percent of the precise answer between roughly 4% and 15%. At very high or very low rates, the approximation drifts further from the exact figure, so use a full compound interest calculation for those cases.
