Rule of 72

The Rule of 72 is a quick mental-math shortcut for estimating how many years it takes an investment to double at a given annual rate of return: divide 72 by the rate. At 8% a year, money doubles in about 9 years (72 divided by 8); at 6%, about 12 years.

It is an approximation, not an exact formula, but it stays reasonably accurate across the rates most long-term investors actually plan around (roughly 4% to 12%), which is why it remains a standard tool for quick compounding intuition without reaching for a calculator.