Calculator
Rule of 72 Calculator
What it calculates
The Rule of 72 is a mental-math shortcut: divide 72 by your annual return rate to estimate the number of years it takes for money to double. It's remarkably accurate for the mid-single-digit to low-double-digit rates most investors encounter.
How this rule of 72 calculator works
This shortcut divides 72 by an annual return rate to estimate the years needed for an investment to double. The calculator also shows the exact compound-growth result for comparison.
Formula
Years to double ≈ 72 ÷ annual return rate
The shortcut is most useful at ordinary investment-rate ranges; accuracy decreases at very high or very low rates.
Primary specifications
Before you use the result
Assumptions
- • The stated rate remains constant through the period.
- • Returns compound and are reinvested.
- • The Rule of 72 is an estimate, not an exact calculation.
Quick start
- 1. Enter an annual return or interest rate.
- 2. Use the estimate for a quick mental-math check.
- 3. Compare it with the exact result before making a financial plan.
Inputs and units
| Input | Unit | Default |
|---|---|---|
| Annual return | percent per year | 8% |
Worked example (hypothetical)
Rule of 72 Calculator: worked example
Hypothetical example using the calculator's default inputs. The numbers are illustrative, not a forecast.
| Annual return | 8% |
|---|
| Rule of 72 estimate | 9.0 years |
|---|---|
| Exact doubling time | 9.0 years |
72 ÷ 8 = 9 years, close to the exact 9.01 years from ln(2) ÷ ln(1.08).
How to interpret the result
Use the estimate as a mental check. The gap between the shortcut and the exact figure widens at high rates, and neither applies if returns are withdrawn or vary from year to year.
Frequently asked questions
How does the Rule of 72 work?
Divide 72 by the annual rate of return (as a whole number). At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years. It's an approximation of the exact logarithmic formula.
How accurate is the Rule of 72?
It's most accurate for rates between about 6% and 10%. For very high or very low rates, the exact compound formula diverges from the estimate.
Does the Rule of 72 work for inflation?
It can approximate how long a steady inflation rate takes to halve purchasing power, but the exact logarithmic calculation is more precise.
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Rule of 72 Calculator shows the arithmetic. Members get the monthly call that decides which inputs matter, plus written breakpoints when the facts move. $39/month or $249/year.
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