Rule of 72 Calculator
Estimate how long money takes to double, or what rate you need to double it in a given time.
What the Rule of 72 Calculator does
Divide 72 by the annual rate and you get a close estimate of the years needed to double your money. It is a mental-arithmetic shortcut for the exact logarithmic formula, and it is accurate to within a few months across typical investment rates.
Formula
Years to double ≈ 72 ÷ rateExact: years = ln(2) ÷ ln(1 + r)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Solve for | one of 2 options | Yes | — |
| Annual interest rate | % | In some modes | Shown Solve for is Years to double. |
| Years available | years | In some modes | Accepts more than 0, up to 100. Shown Solve for is Rate needed. |
How to use it
- Choose Solve for.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
An investment returns 8% a year.
- Mode
- Years to double
- Rate
- 8
72 ÷ 8 = 9 years. The exact answer is 9.006 years.
Frequently asked questions
Why 72 and not 70?
70 is mathematically closer for continuous compounding, but 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, which makes the mental arithmetic far easier.
Does it work for inflation?
Yes, in reverse. At 3% inflation, prices double in about 24 years, meaning your money loses half its purchasing power.