✌️ Rule of 72 Calculator
Enter an annual rate of return to see roughly how long your money takes to double, with the Rule of 72, the Rule of 69.3 and the exact answer side by side.
Quick answer: Years to double ≈ 72 ÷ annual rate. At 8% a year, 72 ÷ 8 = 9.0 years; the exact compound formula gives 9.0 years too (9.006).
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Rule of 72 Calculator inputs
Result
Years to double your money
9 years
at 8% a year (Rule of 72)
| Rule of 72 | 9 years |
| Rule of 69.3 | 8.7 years |
| Exact (compound formula) | 9 years |
- Triples in about
- 14.3 years
- Grows 10× in about
- 29.9 years
Doubling time at common rates
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36 years | 35 years |
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6 years | 6.1 years |
| 15% | 4.8 years | 5 years |
| 20% | 3.6 years | 3.8 years |
How the Rule of 72 works
Under compound growth, money doubles when (1 + r)t = 2. Solving for t gives ln 2 ÷ ln(1 + r), which is close to 72 divided by the percentage rate for typical returns.
Years to double ≈ 72 ÷ annual rate (%) · Exact = ln(2) ÷ ln(1 + rate)
Example: at 8% a year, 72 ÷ 8 = 9 years. The exact result is ln 2 ÷ ln 1.08 = 9.006 years, so the shortcut is almost perfect.
Doubling time at common rates
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 12% | 6.0 years | 6.1 years |
Where to use it
- Investing: estimate how long a portfolio takes to double at a given return.
- Inflation: see how fast purchasing power halves.
- Debt: understand how quickly unpaid balances can double.
- Fees: a 2% yearly fee noticeably lengthens the doubling time.
This is an educational estimate, not financial advice. Real returns vary and are not guaranteed.
Frequently asked questions
What is the Rule of 72?
A mental shortcut: divide 72 by the annual percentage return and you get the approximate number of years for an investment to double with compound growth.
How accurate is the Rule of 72?
It is very accurate for rates between about 6% and 10% and slightly less accurate at very low or very high rates. The calculator shows the exact figure for comparison.
Why 72 and not 70 or 69?
72 has many divisors (2, 3, 4, 6, 8, 9, 12), making mental maths easy, and it fits typical rates of 6–10% best. 69.3 is more precise for continuous compounding.
Can I use it for inflation or debt?
Yes. At 3% inflation prices double in about 24 years, and a debt charging 18% interest doubles in about 4 years if nothing is repaid.
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