%
$
Rule of 72 Estimate
10.3 years
to double at 7% annual return
Precise (Compound)
10.2 years
Accuracy
1.0%
off from precise
Free · Fast · Private
Complete explanation of the Rule of 72, why 72 is used, and when it works best.
Read guide →How compound interest works and its relationship to the Rule of 72.
Read guide →Rule of 72 applied to stock returns, savings accounts, and inflation.
Read guide →The Rule of 72 is a simple formula to estimate how long it takes to double your money at a given annual rate of return. Divide 72 by the interest rate to get the approximate number of years.
The Rule of 72 is most accurate for interest rates between 6% and 10%. For lower rates, Rule of 70 or 69.3 may be more precise. The calculator shows both the Rule of 72 estimate and the precise compound interest calculation for comparison.
Yes! For inflation, divide 72 by the inflation rate to estimate how many years it takes for purchasing power to halve. At 3% inflation, it takes about 24 years for your money to lose half its value.
The natural log of 2 is 0.693, so 69.3 is mathematically exact for continuous compounding. However, 72 has many divisors (2, 3, 4, 6, 8, 9, 12, 18, 24, 36), making mental math easier. It's also a slightly better approximation for typical interest rates (6-10%) where compounding is annual, not continuous.
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Compound interest is interest earned on both your initial investment and the accumulated interest from previous periods. This is why money doubles over time — the interest "compounds," growing your investment faster than simple interest would.
Estimate how your investments will grow by entering your expected rate of return. If you're earning 8% annually, your money doubles every 9 years (72÷8=9). This helps visualize the power of long-term compound growth.
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