Rule of 72
A quick mental-math shortcut for estimating how long it takes an investment to double.
Divide 72 by the annual growth rate to get a rough estimate of the number of years it takes an investment to double. It's an approximation, not an exact formula — it gets less accurate at very high or very low rates — but it's fast enough to do in your head.
Worked Example
At 6% annual growth, 72 ÷ 6 = 12 years to double. At 9%, it's 72 ÷ 9 = 8 years. A precise compound-interest calculation gives a nearly identical answer.
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