The Rule of 72 is a quick mental shortcut for estimating how long a compounded investment takes to double: divide 72 by the expected annual rate of return (as a percentage). For example, at an 8% annual return, money doubles in roughly nine years (72 ÷ 8 = 9).
You can also reverse it—divide 72 by a target number of years—to estimate the average annual return needed to double in that time. The approximation works best for rates roughly in the 6%–10% range and assumes a steady, reinvested return without fees, taxes, or withdrawals.
Source: The Rule of 72 — Investopedia