Estimate how many years it takes for money to double at a given interest rate, with the exact compounding answer for comparison.. Rule of 72 Calculator is a free online tool from the ToolsRift Finance & Health Calculators collection. It runs entirely in your browser — nothing to install, no signup, and your data never leaves your device.
Enter an annual interest or growth rate. The calculator shows the Rule of 72 doubling-time estimate next to the exact compounding answer for comparison.
It is a shortcut: divide 72 by the annual interest rate to estimate the years needed to double your money. At 8%, that is 72 ÷ 8 = about 9 years.
It is a close approximation for rates between roughly 6% and 10%. For very high or low rates the exact logarithmic formula, also shown here, is more precise.
The precise time to double is ln(2) / ln(1 + r), where r is the annual rate. This calculator shows both the Rule of 72 estimate and this exact value.