Rule of 72 (time to double your money)
Find out how long an investment takes to double, triple or grow tenfold at a given interest rate, with the rule of 72 and the exact calculation, or what rate you need to double it in a given time.
How to useHow long until your money doubles
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Rule of 72 (time to double your money): what it is for
Type the annual interest rate and see how many years it takes for money to double with the rule of 72 and with the exact logarithm calculation, plus the error of the rule and the time to triple, quadruple and grow tenfold. In reverse mode, enter the time and find the rate you need to double your money.
The calculations run in your browser and are a simulation only; they are not investment advice.
How to use
- Choose to calculate the time or the rate.
- Type the value.
- Compare the rule with the exact calculation.
When to use it
- Understanding the power of compound interest.
- Comparing investments quickly.
- Seeing the effect of inflation over the years.
Frequently asked questions
What is the rule of 72?
It is a mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes for money to double with compound interest. At 8% a year, 72 ÷ 8 = 9 years. The exact calculation is ln(2) ÷ ln(1 + rate).
Does it work for any rate?
It is very accurate between 6% and 10% a year. For lower rates, the rule of 70 or 69 is closer, and for high rates the error grows. The tool shows the approximate and exact values side by side.
Does it work for inflation?
Yes, the other way around: with 6% inflation a year, prices double in about 12 years, meaning the purchasing power of idle cash is cut in half in that time.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.