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The Rule of 115 is the tripling-time companion to the well-known Rule of 72 — a quick mental-math shortcut for estimating how long it takes an investment to triple at a given annual return.
This calculator applies the Rule of 115, plus shows the exact figure using logarithms for comparison.
How it works
Enter an annual interest rate. The calculator divides 115 by that rate to estimate the years to triple, and separately computes the exact years using logarithms so you can see how close the shortcut gets.
- Enter annual interest rate (%).
- Click Calculate to see your results.
Examples
6% annual return
At a 6% annual return, the Rule of 115 estimates about 19.2 years to triple an investment — close to the exact figure of 18.9 years.
Who should use it
- Quickly estimating investment tripling time without a calculator.
- Comparing the growth speed of investments at different rates.
Industry applications
- Personal investing and financial planning
- Financial education
Advantages
- Extremely quick mental-math shortcut once you know the rate.
- Shown alongside the exact logarithmic answer for comparison.
Limitations
- Accuracy decreases at very low or very high interest rates.
Common mistakes to avoid
- Treating the Rule of 115 estimate as exact rather than a quick approximation — use the exact logarithmic figure when precision matters.
- Applying the rule to a rate that includes inflation without separately accounting for real versus nominal returns.
Best practices
- Use the Rule of 115 for quick mental estimates and sanity checks, and the exact logarithmic figure for anything requiring precision.
Tips
- Pair this with the Rule of 72 (doubling time) to build a quick mental sense of how a given rate compounds over different multiples.