By ToolzHive
About this tool
The Rule of 72 is a quick way to estimate how long an investment takes to double: divide 72 by the annual interest rate. This calculator shows the Rule of 72 estimate alongside the exact answer for yearly compounding, the Rule of 70 and 69.3 variants, and how long it takes to triple. You can also work backwards to find the rate needed to double your money in a set time.
How to use Rule of 72 Calculator
- Choose whether to find the years to double or the rate needed.
- Enter the annual rate of return, or the number of years.
- Compare the quick estimate with the exact result.
Worked example
At 8% a year, money doubles in about 72 ÷ 8 = 9 years. The exact figure with yearly compounding is 9.01 years, so the rule is very close.
How accurate is the Rule of 72?
It is very accurate for rates between about 6% and 10%. For lower or higher rates, the exact calculation shown by the calculator is better.
What are the Rule of 70 and 69.3?
They are variants. 69.3 is exact for continuous compounding, and 70 is easier to use with low rates, such as estimating inflation effects.
Can I use it for inflation?
Yes. At 4% inflation, prices double in about 18 years, meaning money loses half its buying power in that time.
Put this guide into practice
- Rule of 72 Calculator — Estimate how long it takes to double your money at a given rate.