Rule of 72 calculator
Divide 72 by an annual return in percent to estimate how long a balance takes to double. The page also shows the exact compound-interest doubling time at that same rate. The Rule of 72 guide uses the same example.
Years to double ≈ 72 ÷ annual return in percent. Exact years = natural log of 2 ÷ natural log of (1 + annual rate).
Rule of 72
Enter an annual return and a starting amount.
How to estimate a doubling time
- Enter an annual return. Use percent, so 8 means 8% a year. The rate stays constant for every doubling.
- Compare the shortcut with the exact years. The Rule of 72 divides 72 by the percent. The exact line uses compound interest. The gap is the shortcut error.
- Read the milestone balances. Each milestone applies the exact doubling time to the starting amount. The balance doubles, then doubles again.
Frequently asked questions
- How does the Rule of 72 work?
- Divide 72 by the annual return in percent. At 8%, 72 ÷ 8 is 9 years. The exact compound-interest doubling time at 8% is also about 9.01 years. The shortcut is closest for rates near 8%.
- What is the exact doubling time?
- Exact years equal the natural log of 2 divided by the natural log of 1 plus the annual rate. The calculator shows that figure next to the Rule of 72 so you can see the gap at the rate you typed.
- What if the return is 0%?
- A zero return never doubles a positive balance under this model. A negative return is outside the shortcut. Enter a return greater than zero and below 500%.
- Is this investment advice?
- No. The calculator is for general informational and educational purposes only. A doubling time describes a constant rate. It does not predict the return of any security.
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