Transparent method
The formula
Worked example
Example inputs
How to interpret the result
The Rule of 115 gives a fast mental estimate for tripling money, the way the Rule of 72 does for doubling, and both trade precision for speed. At low, steady rates the approximation lands close to a full compound-interest answer; push the rate higher and the shortcut drifts further away, because the rule is a linear stand-in for something that actually compounds.
Before you rely on it
What to check
Cross-check the estimate against a full compound-growth calculation whenever the rate is unusually high or the horizon runs long. The rule is for a quick gut check, not the final number in a real plan.
The common error
Where people go wrong with rule of 115 calculator
Treating the rule's output as an exact figure to build a retirement plan around. It was built for a fast mental estimate, and a real financial decision deserves the actual compounding math, not the shortcut that approximates it.
Sensitivity evidence
How annual return or interest rate changes the estimated tripling time
| Annual return or interest rate | Estimated tripling time | Rule-of-72 doubling time | Entered annual rate |
|---|---|---|---|
| 4.0% | 28.8 years | 18.0 years | 4.0% |
| 6.0% | 19.2 years | 12.0 years | 6.0% |
| 8.0%worked example | 14.4 years | 9.0 years | 8.0% |
| 10.0% | 11.5 years | 7.2 years | 10.0% |
| 12.0% | 9.6 years | 6.0 years | 12.0% |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Annual return or interest rate | 8.0% |
| Estimated tripling time | 14.4 years |
| Rule-of-72 doubling time | 9.0 years |
| Entered annual rate | 8.0% |
Where the inputs come from
Enter the annual return or interest rate as a percentage. The rule of 115 estimates the years needed to triple money by dividing 115 by the rate, and it works best for rates between roughly 5% and 15%. Use a realistic return after fees for the investment in question. No principal is needed, since the answer does not depend on the amount, only on the rate.
When the result stops applying
The rule is an approximation and loses accuracy at very low and very high rates. It assumes constant compounding and no contributions or withdrawals. Fees, taxes and inflation slow the real pace, so a tripling in nominal terms may be less in purchasing power. For exact numbers use a calculator with the actual horizon.
What to compare it with
Use the rule of 72 calculator for doubling, and the future value calculator for exact results. The compound interest page includes deposits. If you want to know how long a goal takes at a given return, the present value and retirement goal calculators are more precise.
People also ask
Frequently asked questions
Where does 115 come from?
It comes from the natural logarithm of 3, about 110 for continuous compounding, and is nudged up to 115 so the estimate fits annual compounding at common rates.
How long does it take to triple at 7%?
About 16.4 years by the rule, since 115 divided by 7 is 16.4. The exact figure for annual compounding is 16.2 years.
Can I use the rule for inflation?
Yes. Divide 115 by the inflation rate to see how long prices take to triple, or how long purchasing power takes to fall to a third.