Rule of 72 Calculator
Estimate how many years it takes to double your money at a given rate of return.
Years to double
9 years
Estimated using the Rule of 72 at an annual return of 8%.
Years to quadruple
18 years
Roughly twice the doubling time.
Key insight
- Estimated Doubling Time
- 9 yrs
- Estimated Quadrupling Time
- 18 yrs
- Possible Doublings in 30 Years
- 3.3
| Annual rate | To double | To quadruple |
|---|---|---|
| 2% | 36 yrs | 72 yrs |
| 4% | 18 yrs | 36 yrs |
| 6% | 12 yrs | 24 yrs |
| 8% | 9 yrs | 18 yrs |
| 10% | 7.2 yrs | 14.4 yrs |
| 12% | 6 yrs | 12 yrs |
How it's calculated
Formula
Years to double ≈ 72 / rateHow it works
The Rule of 72 is a mental-math shortcut derived from the compound-interest formula: it estimates how long an investment takes to double without any logarithms. Just divide 72 by the annual rate. It's most accurate for the returns most investors see — roughly 5% to 12% — where it lands within a fraction of a year of the exact figure. Treat it as a quick tool for financial planning and comparison rather than a precise forecast.
Variables
- rate
- Annual rate of return, as a whole number. Example: 8 for 8%
- Years to Double
- Estimated time for an investment to double.
- Years to Quadruple
- Approximate time needed for an investment to grow fourfold (roughly twice the doubling time).
Worked example
Step 1 Annual Return 8% ↓ Step 2 Apply the Rule of 72 72 ÷ 8 = 9 years ↓ Step 3 Estimate Quadrupling 9 × 2 = 18 years Final Result At an 8% annual return, an investment is expected to double in approximately 9 years and quadruple in about 18 years.
Good to know
When Should You Use the Rule of 72?
- Estimating how quickly an investment could grow
- Understanding how inflation erodes purchasing power over time
- Comparing investment opportunities at a glance
- Quick mental calculations without a calculator
Frequently asked questions
How accurate is the Rule of 72?
Very close for rates between about 5% and 12%. At very high or very low rates the estimate drifts slightly from the exact figure, but it's ideal for quick sanity checks.
Does it work for inflation too?
Yes. Divide 72 by an inflation rate to estimate how long until prices double — and your money's purchasing power halves.
Can I use it to find the rate I need?
Yes — flip it around. To double in a target number of years, divide 72 by that number. To double in 8 years you'd need about a 9% return.
Why 72 and not another number?
72 is close to the exact value (about 69.3) but divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which makes the mental math easy.
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