The Rule of 72 Calculator helps you estimate how long it takes to double your investment. Enter an annual interest rate to find the years to double, or enter a number of years to find the required interest rate. You also get the precise compound interest result alongside the Rule of 72 estimate for comparison.
Rule of 72 Estimate
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Precise Compound Interest Result
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The Rule of 72 is a simple formula used to estimate how long it takes for an investment to double at a fixed annual interest rate. You divide 72 by the annual interest rate to get the approximate number of years. For example, at 8% interest, it takes roughly 72 ÷ 8 = 9 years to double your money.
The Rule of 72 is a close approximation but not perfectly precise. It works best for interest rates between 6% and 10%. For rates outside this range, the estimate may deviate more noticeably from the exact compound interest calculation. This calculator shows both the Rule of 72 estimate and the precise result so you can compare them.
The exact doubling time is calculated using compound interest: t = ln(2) / ln(1 + r), where r is the decimal form of the annual interest rate and ln is the natural logarithm. The Rule of 72 approximates this by using 72 / rate (in percent).
Yes. If you know how many years you want to double your investment, divide 72 by the number of years to find the approximate annual interest rate needed. For example, to double your money in 6 years, you need roughly 72 ÷ 6 = 12% annual return.
The mathematically derived constant is closer to 69.3 (since ln(2) ≈ 0.693). However, 72 is preferred because it is divisible by more integers (1, 2, 3, 4, 6, 8, 9, 12), making mental math easier. At typical interest rates around 8%, 72 also gives a more accurate approximation than 69.
Yes. The Rule of 72 can be applied to any quantity that grows at a fixed compound rate — such as inflation, GDP growth, population growth, or even the erosion of purchasing power. For example, at 4% inflation, the cost of goods doubles in roughly 18 years.
The Rule of 72 is most accurate for annual interest rates in the range of 6% to 10%. At very low rates (under 2%) or very high rates (above 30%), the approximation becomes less precise compared to the exact compound interest formula.
The Rule of 72 is derived from compound interest, not simple interest. It assumes interest is compounded annually. Simple interest does not produce exponential growth, so the Rule of 72 would not apply in that case.