Rule of 72
Planning & BudgetingA shortcut to estimate how long an investment takes to double: divide 72 by the annual return rate (e.g., 72 ÷ 8% = 9 years).
Example
Example: Your investment earns 8% annually. 72 / 8 = 9 years to double. $10,000 becomes $20,000 in 9 years, $40,000 in 18 years, $80,000 in 27 years, and $160,000 in 36 years — all without adding a single dollar. In a savings account at 4%, doubling takes 18 years. The Rule of 72 is the fastest way to estimate compound growth mentally.