The Rule of 72 is a quick mental-math formula for estimating how many years an investment needs to double in value at a fixed annual rate of return.
The Formula: Years to Double = 72 / Annual Interest Rate
Examples
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6% return: 72 / 6 = about 12 years to double
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9% return: 72 / 9 = about 8 years to double
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Reverse it: to double in 10 years you need roughly a 7.2% annual return
Why 72 works
72 is a convenient approximation of the natural-log math behind compound growth — and it divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which is what makes it a mental-math tool.
Community note: the rule is least accurate at very high rates — above ~20%, use 78 instead. Edited with the Drive editor.