Try these example inputs in the Rule of 72 calculator.
These figures match this guide's worked example. They are illustrative. They stay on this device and never appear in the URL.
The Rule of 72 is a shortcut for how long a balance takes to double at a constant annual rate. Divide 72 by the rate in percent. At 8% a year, 72 ÷ 8 is 9 years.
Key takeaways
Years to double ≈ 72 ÷ annual return in percent
Exact years = natural log of 2 ÷ natural log of (1 + annual rate)
The rate stays constant, and the balance compounds once a year. No contributions are added along the way.
Take an 8% annual return and a $10,000 start.
Each step applies the exact doubling time again. The Rule of 72 calculator prints the shortcut and the exact years side by side.
The shortcut is a neighborhood trick, not a law. Error is (shortcut − exact) ÷ exact:
| Annual rate | Shortcut (72 ÷ rate) | Exact years | Error |
|---|---|---|---|
| 1% | 72.0 | 69.66 | +3.4% |
| 8% | 9.0 | 9.01 | −0.1% |
| 12% | 6.0 | 6.12 | −1.9% |
| 24% | 3.0 | 3.22 | −6.9% |
Near 8% the gap is tiny. Far from 8% it is not. At 0% a positive balance never doubles under this model; the shortcut is undefined because you cannot divide by zero.
It turns a rate into a span of years you can check in your head. Use the exact line when the rate is far from 8%, or when you want the milestones on a starting amount.
Contributions, withdrawals, taxes, and a rate that changes are all outside the shortcut. It also describes a smooth path. A balance that doubles over nine bumpy years still doubled. The rule does not show the bumps.
If you want the balance after a contribution schedule, use compound growth. Check the shortcut against the exact years in the Rule of 72 calculator. At 8% both answers are about 9 years.
For general education only. Nothing here is investment advice.
Formulas and worked examples are MarketCapLens's. See who checks the numbers.