Glossary term
Rule of 72
The Rule of 72 is a quick estimate for how long it may take money to double by dividing 72 by an assumed annual return.
Updated
Read time
What Is the Rule of 72?
The Rule of 72 is a quick estimate for how long it may take money to double at a fixed annual return. It is not exact, but it gives readers a simple way to connect return, time, and compounding without building a full spreadsheet.
The shortcut is most useful as a mental model. It helps show why small differences in return can matter over long periods and why compounding needs time to do its work.
Key Takeaways
- The Rule of 72 estimates doubling time.
- The common shortcut is 72 divided by the annual return percentage.
- It works best as a rough estimate, not a precise projection.
- The rule is tied to compound interest.
- It can also estimate the return needed to double money over a chosen number of years.
Rule of 72 Formula
The common Rule of 72 formula is:
In the formula, annual return percent is written as a whole number. For example, use 6 for 6 percent, not 0.06.
How the Rule of 72 Works
If money earns about 6 percent a year, the Rule of 72 estimates that it may double in roughly 12 years because 72 divided by 6 equals 12. If the assumed return is 8 percent, the estimate falls to about 9 years.
The same relationship can be flipped. If someone wants to estimate the return needed to double money in 10 years, 72 divided by 10 gives about 7.2 percent.
Why the Rule Is Only an Estimate
The Rule of 72 assumes a fixed return and clean compounding. Real investments do not move in a straight line. Fees, taxes, inflation, contribution timing, and market volatility can all change the result.
That does not make the rule useless. It makes it a rough planning shortcut rather than an investment forecast.
How It Connects to Future Value
The Rule of 72 is a simplified version of a future-value idea. It estimates how long it may take a starting amount to become twice as large. For more exact calculations, use future value and compound-interest math.
The Bottom Line
The Rule of 72 is a quick way to estimate how long money may take to double at a fixed annual return. It is useful for understanding compounding, but it should be treated as a rough shortcut, not a guarantee.