Estimate how long it takes to double your investment using the Rule of 72. Enter an annual interest rate to see the approximate doubling time. Reverse mode also available. Pure client-side, instant results.
The Rule of 72 is a simple mental math shortcut used in finance to estimate how long an investment will take to double at a given annual rate of return. Just divide 72 by the interest rate percentage. For example, at 8% annual return, your money doubles in approximately 9 years (72 ÷ 8 = 9). It also works in reverse: divide 72 by your target years to find the required rate. While not perfectly precise, it is remarkably accurate for rates between 6% and 10%, and incredibly useful for quick financial planning.
The Rule of 72 is a quick mental calculation used to estimate the number of years required to double an investment at a given annual rate of return. You simply divide 72 by the interest rate percentage.
The Rule of 72 is most accurate for interest rates between 6% and 10%. At very low or very high rates, the approximation deviates slightly from the exact logarithmic calculation. This calculator shows both the Rule of 72 estimate and the exact value for comparison.
Yes! You can use the Rule of 72 to estimate how long it will take for inflation to halve the purchasing power of money. Just divide 72 by the inflation rate. For example, at 3% inflation, purchasing power halves in about 24 years.
The exact doubling time is calculated using natural logarithms: t = ln(2) / ln(1 + r), where r is the decimal interest rate. The Rule of 72 approximates this without requiring a calculator.
The Rule of 72 works reasonably well for most common investment returns (2% to 15%). For very high rates (above 20%), the Rule of 69 or 69.3 is slightly more accurate.