A quick mental shortcut for how long it takes money to double: divide 72 by your interest rate. See how accurate that shortcut really is, and watch your money double in real time.
| Rate | Rule of 72 Estimate | Precise Years | Difference |
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The Rule of 72 is a mental math shortcut: divide 72 by an annual interest or growth rate to estimate how many years it takes an investment to double. At 8 percent, that's 72 divided by 8, or 9 years. It works because of the mathematical properties of compound growth, and the number 72 was chosen specifically because it divides evenly by so many common numbers, including 2, 3, 4, 6, 8, 9, and 12, making it easy to compute in your head. Every slider on this page has a matching number field: drag to explore, or type exact figures if you already know your numbers.
The exact number of years to double an investment is the natural log of 2 divided by the natural log of one plus the rate. That calculation isn't something most people can do in their head, which is exactly why the Rule of 72 exists as an approximation. The constant that would make the shortcut mathematically perfect at every rate is closer to 69.3, but 72 is used instead because it's far more divisible by whole numbers, trading a small amount of precision for a large amount of everyday usability.
The Rule of 72 is most accurate in the 6 to 10 percent range, which happens to cover typical long run stock market returns, which is likely why it became so popular in personal finance. At very low rates, like 2 percent, the estimate overstates the time to double by about a full year. At very high rates, like 20 percent or more, it understates the time to double, also by around a year or more. The table below shows this pattern precisely across a range of rates.
The same shortcut works in reverse: divide 72 by the number of years you want your money to double in, and the result is roughly the annual return you'd need. It's also commonly applied to inflation, estimating how many years it would take prices, or the purchasing power lost to inflation, to double at a given inflation rate, which is a sobering way to visualize inflation's long run effect.
The Rule of 72 is frequently tested as a quick estimation tool on the SIE exam and other FINRA licensing exams, and the underlying compound growth concept is foundational to time value of money content on CFA Level 1 and the CFP exam. Our TVM Solver can compute the precise, non approximated version of this same question.
The Rule of 72 assumes annual compounding. More frequent compounding, such as monthly or daily, will cause an investment to double slightly faster than either the rule or the annual precise formula suggests, though the difference is usually small at typical rates.
Yes. The Rule of 69.3 is the mathematically exact constant for continuous compounding, and the Rule of 70 is sometimes used as a slightly more accurate but less divisible alternative to 72. Rule of 72 remains the most widely taught version because of how easily 72 divides by common rates.
Not directly with 72. A commonly cited approximation for tripling time uses 114 divided by the rate, following the same logic but calibrated to a factor of three instead of two.
Yes. Each slider has a paired number input. Type directly into any gold field to enter a precise figure, and the slider along with all results update immediately.
The Rule of 72 shows up directly on the SIE and other FINRA exams. Practice for free.
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