Rule of 72 Calculator.
Enter an expected return and see how many years it takes to double your money — the famous Rule of 72 shortcut alongside the mathematically exact answer.
Before you run the numbers.
The Rule of 72 is the single most useful piece of mental math in personal finance: divide 72 by your annual return, and you get roughly the number of years it takes your money to double. At 8%, that's 9 years. At 6%, 12 years. It's the trick nearly every investing book teaches because it makes compounding tangible without a spreadsheet.
This calculator does the shortcut for you — and then shows you the exact doubling time (ln 2 ÷ ln(1 + return)) so you can see how close the approximation really is. Spoiler: it's closest around 8%, where the error is less than a week, and drifts slightly at the extremes.
Use it to reason quickly: how many times will my money double before I retire? Enter a starting amount and a horizon and the calculator counts the doublings and projects the ending balance — the same exponential math the compound interest calculator runs, boiled down to its most memorable form.
Run your scenario.
See a worked example
At an 8% return, the Rule of 72 says your money doubles in 72 ÷ 8 = 9 years.
Rule of 72: 72 / 8 = 9.00 yrs
Exact: ln(2) / ln(1.08) = 0.693147 / 0.076961 = 9.01 yrs
The shortcut is off by less than a week at 8% — that's why it survives. It drifts a bit at very high or very low rates (use the exact figure there), but for back-of-envelope planning, 72 ÷ your return is close enough to trust.
Educational tool, not financial advice. Doubling times assume a constant, hypothetical annual return — real returns vary and are never guaranteed. This ignores taxes, fees, and inflation. Consult a licensed financial professional before making investment decisions.
Taught in these books.
Common questions.
Why 72 and not some other number?
72 is a convenient, highly divisible number (2, 3, 4, 6, 8, 9, 12 all divide it cleanly) that happens to approximate the exact doubling formula very well for the mid-single-digit returns most people plan around. The mathematically 'perfect' constant is closer to 69.3 for continuous compounding, but 72 is easier to divide in your head and is more accurate for the annually-compounded returns real investors actually earn.
When is the Rule of 72 inaccurate?
It's most accurate near an 8% return. Below ~4% it slightly underestimates the doubling time, and above ~15% it slightly overestimates it. For those cases, use the 'exact doubling time' figure this calculator shows. For everyday planning at 6–10%, the shortcut is close enough to trust.
Does a faster doubling time mean a better investment?
Not by itself. A higher assumed return doubles your money faster on paper, but higher expected returns come with higher risk and more volatility — and the return is hypothetical, not guaranteed. The Rule of 72 tells you the math of compounding; it says nothing about whether a given return is realistic or safe.