Rule of 72 Calculator
Estimate doubling time with the Rule of 72.
How fast money doubles
The Rule of 72 is a mental-math shortcut: divide 72 by the annual return to estimate years to double. At 8%, money doubles in about nine years — close to the exact figure without a calculator.
It also works for inflation, showing how fast prices double and buying power halves — at 3% inflation, money loses half its value in about 24 years.
An approximation
It is most accurate for rates between 6% and 10%; extremes drift. Your inputs never leave your device.
Frequently asked questions
How does the Rule of 72 work?
Divide 72 by the annual rate to estimate the years it takes money to double. At 8%, $10,000 becomes $20,000 in about 9 years.
How accurate is the Rule of 72?
Very close for rates between about 6 and 10%. At 8%, the exact doubling time is 9.01 years; at very high or very low rates, the estimate drifts.
Can it estimate how inflation shrinks money?
Yes. At 3% inflation, prices double in about 24 years, meaning money loses half its buying power in that time.
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