Albert Einstein allegedly called compound interest "the most powerful force in the universe," and the math supports this claim. At an 8% annual return, money doubles approximately every 9 years according to the Rule of 72. This means $10,000 becomes $20,000 in 9 years, $40,000 in 18 years, $80,000 in 27 years, and a remarkable $160,000 in 36 years - all without adding a single additional dollar to your initial investment.