The Rule of 72 Explained
The Rule of 72 is a fast shortcut for estimating how long it takes money to double. Divide 72 by the annual rate, and you get an approximate doubling time in years.
Why it works
The Rule of 72 is based on compound growth. It is not exact, but it is close enough for rough planning and quick comparisons.
Quick table
| Rate | Estimated doubling time |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
Where it helps
It is useful when you want to compare rates quickly without opening a spreadsheet. It also makes the cost of high-interest debt feel more real because you can estimate how fast a balance can double.
Where it falls short
- It is an estimate, not an exact forecast.
- It does not handle monthly contributions.
- It is less useful for unusual rates or changing returns.
Rule of 72 vs calculator
The shortcut is great for intuition. A real calculator is better when you want precise numbers, contribution modelling, or a comparison between monthly and yearly compounding.
If you like finding practical tools beyond finance, BetterThan.Tools is a useful directory to browse.
Use the Rule of 72 calculator
Check doubling time with your own rate›
FAQ
How accurate is the Rule of 72?
It is good for rough planning, especially at common long-term return ranges, but it is not exact.
Can I use it for debt too?
Yes. It can help you see how quickly a high-interest balance might double.
When should I use a calculator instead?
Use a calculator when you want precision, contributions, or a richer comparison than a mental shortcut can provide.