What's the rule of 72 in a bank?

Written by Editorial Team | Last Updated: August 2026

The Rule of 72 is a simplified mental math formula used by bankers, financial planners, and consumers to quickly estimate how long it will take for an investment or a bank deposit to double in value at a fixed annual interest rate. By dividing the number 72 by the annual percentage rate of return, you receive an estimated timeline in years. For example, if a savings account yields four percent interest, dividing 72 by four reveals that the deposited money will take approximately eighteen years to double. While it serves as a helpful shortcut for compound interest calculations, it functions best for interest rates ranging between six and ten percent.

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