What is the Rule of 72?
The Rule of 72 is a simple formula to estimate the number of years required to double your investment at a given annual rate of return. Divide 72 by the annual rate of return to get the approximate number of years.
How to Use the Rule of 72 Calculator
Simply enter the annual interest or return rate. The calculator instantly tells you how many years it will take for your investment to double. Use it to quickly compare different investment options — for example, at 6% your money doubles in 12 years, but at 9% it doubles in just 8 years.
The Rule of 72 Formula
Years to Double = 72 ÷ Annual Interest Rate. This simple formula is remarkably accurate for rates between 2% and 15%. For example: at 8% annual return, 72 ÷ 8 = 9 years to double. The exact formula uses natural logarithms (ln(2) / ln(1 + r)), but the Rule of 72 provides a quick mental math shortcut that investors have used for centuries.
Frequently Asked Questions
Why is 72 used and not another number?
72 is used because it closely approximates the exact doubling time for typical interest rates (2-15%) and is easily divisible by many common numbers (2, 3, 4, 6, 8, 9, 12). For very low rates below 2%, the Rule of 70 is slightly more accurate, and for rates above 20%, the Rule of 78 works better.
Can the Rule of 72 be used for things other than investments?
Yes. It works for anything that grows at a compound rate. You can use it to estimate how quickly prices double with inflation (at 3% inflation, prices double in 24 years), how fast a country's GDP doubles, or how quickly debt grows if unpaid. It is a universal tool for understanding exponential growth.