Compound interest guide
Rule of 72 Examples for Fast Investment Estimates
Use the Rule of 72 to estimate doubling time, compare returns, and understand how fees slow long-term wealth growth.
Try this scenario
Open the calculator with this guide's example already filled in, then adjust the monthly contribution, return, or timeline to compare your own scenario.
Open calculator exampleKey Takeaways
- Divide 72 by the annual return to estimate doubling time.
- The rule is a quick approximation, not a precise forecast.
- Small fees can add years to the time required to double money.
Example Scenarios
| Scenario | Principal | Monthly | Return | Years | Result |
|---|---|---|---|---|---|
| 8% return | $10,000 | $0 | 8% | 9 | Roughly doubles to $19,990 |
| 6% return | $10,000 | $0 | 6% | 12 | Roughly doubles to $20,122 |
The shortcut
The Rule of 72 estimates how long it takes money to double at a fixed annual return. Divide 72 by the annual rate. At 8%, the estimate is 72 divided by 8, or about 9 years.
The rule is useful because it turns abstract percentages into time. A 4% return doubles in about 18 years. A 9% return doubles in about 8 years. A 12% return doubles in about 6 years.
What it is good for
Use it for quick comparisons. If one account yields 4% and another long-term investment is expected to return 8%, the second does not merely earn twice as much each year. It roughly halves the doubling time.
The rule also makes fees visible. If an investment earns 8% before fees and costs 1% per year, the net return is 7%. The doubling estimate moves from 9 years to about 10.3 years.
Where it breaks down
The Rule of 72 assumes a steady annual rate. Real investments do not move in a straight line. Returns can be negative for a year or several years, then recover later.
It is also less accurate at very low or very high rates. For financial planning, use it as a mental model first, then use the calculator for a more precise estimate.
Try the example
The linked calculator example uses $10,000 at 8% for 9 years with annual compounding. The final balance is close to $20,000, which matches the Rule of 72 estimate.
Questions
Can I use the Rule of 72 with monthly contributions?
Not directly. It estimates doubling for a single balance. For recurring deposits, use the full compound interest calculator.
Why 72 instead of 70?
72 is easy to divide by many common rates and gives a useful approximation across typical investment return ranges.