Rule of 72 Calculator
Quickly estimate how many years it takes to double your money at a given interest rate.
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Time to double your money
How the Rule of 72 works
The Rule of 72 is a quick mental-math shortcut for estimating how long an investment takes to double at a fixed annual rate of return, with no calculator required: just divide 72 by the interest rate. At 8% annual growth, for example, 72 ÷ 8 = 9 years to double. It's an approximation of the exact compound interest formula, but it stays remarkably accurate for rates roughly between 6% and 10% - the range most long-term investment and savings discussions fall into.
FAQ
How accurate is the Rule of 72?
Very close for rates between about 6% and 10%, usually within a few weeks of the exact answer. At much higher or lower rates it drifts further off - for very precise numbers at unusual rates, use the "exact doubling time" figure above, which comes from the actual compound interest formula rather than the shortcut.
Does the Rule of 72 work for debt, not just investments?
Yes - the same math applies in reverse. It estimates how quickly a balance doubles at a given interest rate, which is just as relevant for high-interest debt as it is for investments. See the debt payoff calculator for a full payoff timeline instead of just a doubling estimate.
How is this different from the compound interest calculator?
This tool answers one specific question fast - "how long until this doubles?" - using a well-known shortcut. The compound interest calculator is for full projections with contributions, compounding frequency, and a custom timeframe.
💡 Did you know?
The Rule of 72 traces back to 1494, when Italian mathematician Luca Pacioli documented it in his book Summa de Arithmetica. He didn't claim to invent it - he described it as a shortcut merchants already used, suggesting it was common knowledge among Italian traders long before he wrote it down.
The math behind it comes from the natural logarithm of 2 (≈0.693), which is the "true" constant for continuous compounding - that would make 69.3 the mathematically precise number to use. 72 was chosen instead because it divides evenly by more numbers (2, 3, 4, 6, 8, 9, 12), trading a little precision for much easier mental math.
