Rule of 72 Calculator

The Rule of 72 is a classic, back-of-the-envelope shortcut used in finance to quickly estimate how long an investment takes to double under a fixed annual compound rate of interest.

How the Rule of 72 Works

The shortcut formula is incredibly simple:

Years to Double ≈ 72 ÷ Annual Interest Rate

For example, if you earn an 8% return annually, it will take approximately 9 years (72 ÷ 8 = 9) to double your money. If you earn 6%, it takes approximately 12 years (72 ÷ 6 = 12).

Rule of 72 vs. Exact Compounding

While the Rule of 72 is an excellent mental approximation, it is not mathematically perfect. The exact mathematical doubling time is calculated using logarithms:

Exact Years = ln(2) ÷ ln(1 + r)

Where **r** is the decimal representation of the interest rate (e.g. 0.08 for 8%). Our calculator runs both calculations side-by-side so you can see exactly how close the estimate matches real-world math.