⏳ Rule of 72 Calculator
A quick mental-math shortcut for estimating how fast money doubles.
How This Is Calculated
Years to Double ≈ 72 ÷ Interest Rate. Example: at 8% annual return, money doubles in about 72 ÷ 8 = 9 years. This calculator also shows the exact answer (using the precise compound interest formula) alongside the quick Rule of 72 estimate.
Frequently Asked Questions
How accurate is the Rule of 72?
It's a close approximation for rates between roughly 6% and 10% — slightly less precise outside that
range, where the exact formula gives a more accurate answer. For quick mental math, it's remarkably reliable.
Guide: understanding your result
- Rule of 72: years to double ≈ 72 ÷ annual growth rate (in percent).
- It is a quick mental estimate, most accurate for rates between about 6% and 10%.
- It works for anything that compounds — including inflation: at 3% inflation, prices double in about 24 years.
Rule of 72 vs exact doubling time
| Annual rate | Rule of 72 | Exact (years) |
|---|---|---|
| 2% | 36.0 | 35.0 |
| 4% | 18.0 | 17.7 |
| 6% | 12.0 | 11.9 |
| 8% | 9.0 | 9.0 |
| 10% | 7.2 | 7.3 |
| 12% | 6.0 | 6.1 |
Common mistakes to avoid
- Using it for very high rates, where it becomes less accurate.
- Treating an assumed rate as a promised return.
Frequently asked questions
Does this tell me what to invest in?
No. It only estimates doubling time for whatever rate you enter; it is not investment advice.
Official sources
Examples are calculated live from the same data the calculator uses and are for education only — they are not financial, tax or investment advice.
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