Rule of 72 Calculator

About this calculator

This calculator applies the Rule of 72, a mental-math shortcut that estimates how many years it takes an investment to double by dividing 72 by the annual rate of return — at 8% growth, for instance, 72 ÷ 8 gives roughly 9 years to double, without needing to solve the exact compound interest formula.

It's a favorite of investors and financial educators precisely because it's fast and close enough for real decisions: comparing how a 4% savings account stacks up against a 7% index fund, or getting an intuitive sense of what a country's inflation rate is doing to purchasing power over time (the same shortcut works in reverse to estimate how fast money loses half its value). It won't replace an exact calculation for a legal document, but for a quick gut-check on compounding, it's the tool people reach for first.