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Rule of 72 Calculator

Quickly estimate how many years it takes for an investment to double using the Rule of 72, along with the exact figure from the compound interest formula.

The expected annual rate of return (%%).
Enter any amount to see its doubled value.

Results

Approx. Years to Double (Rule of 72)0.0 years
Exact Years to Double0.00 years
Doubled Amount$20,000.00

📖What is it?

The Rule of 72 is a mental-math shortcut: divide 72 by the annual interest rate to estimate how many years it takes for money to double. This calculator also computes the exact doubling time using the compound interest formula ln(2) / ln(1 + r).

🎯How to use

1. Enter the annual interest rate or expected rate of return. 2. Optionally enter a starting amount to see the doubled value. The calculator instantly shows both the Rule of 72 approximation and the exact compound-growth answer.

💡Example scenario

At an 8% annual return, the Rule of 72 estimates doubling in 72 / 8 = 9.0 years. The exact calculation gives 9.01 years — remarkably close! If you start with $10,000, it will grow to $20,000 in about 9 years.

🏆Pro tip

The Rule of 72 is most accurate for rates between 6% and 10%. For very low or very high rates, use the exact formula instead. You can also use it in reverse: if you want to double your money in 6 years, you need a rate of about 72 / 6 = 12%.