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Years to Double
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Quickly estimate how long it takes an investment to double, or the rate you'd need, using the classic Rule of 72 shortcut.
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Divide 72 by the annual interest rate to estimate the number of years it takes an investment to double. At 8% annual growth, 72 / 8 = 9 years to double.
Years to Double ≈ 72 / Interest Rate
The Rule of 72 is a mental-math shortcut for estimating how long it takes money to double at a fixed annual compound rate, without needing a calculator or the precise logarithmic formula. It's widely used for quick comparisons: at a glance, you can see that money growing at 6% doubles roughly every 12 years, while money growing at 12% doubles in about 6 years.
At an 8% annual return: Years to Double = 72 / 8 = 9 years
The precise formula (ln(2) / ln(1.08)) gives 9.006 years — the Rule of 72 is accurate to within a small fraction of a year at this rate.
72 was chosen historically because it divides evenly by many common small numbers (1, 2, 3, 4, 6, 8, 9, 12), making the mental math easy, and because it happens to produce a close approximation to the true compounding formula specifically in the moderate rate range most savers and investors actually encounter, roughly 6% to 10%. Outside that range, the approximation drifts further from the precise answer, though it remains a reasonable estimate for quick comparisons.
The same shortcut works backward: divide 72 by your target number of years to estimate the annual rate of return you'd need. Wanting to double your money in 6 years, for example, implies you'd need roughly a 12% annual return (72 / 6 = 12), which is a useful sanity check for whether a savings goal's timeline is realistic given typical investment returns.
The Rule of 72 is a quick mental-math shortcut for estimating how long it takes an investment to double at a fixed annual compound rate: divide 72 by the interest rate to get the approximate number of years.
It's a close approximation for rates roughly between 6% and 10%, and becomes less precise at very low or very high rates. For an exact answer, use the precise logarithmic formula rather than the shortcut.
Yes. Divide 72 by the number of years you want your money to double in, and the result is the approximate annual rate of return required.
Want the exact figure instead of an estimate? Use the Compound Interest Calculator or the CAGR Calculator.
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