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What Is Compound Interest?
Compound interest means you earn interest on your interest, not just your principal. Over time, this creates exponential growth — Einstein reportedly called it the "eighth wonder of the world."
What is the Rule of 72?
Divide 72 by your annual return rate to estimate how many years it takes to double your money. At 8% return, your money doubles every 9 years (72÷8=9).
What return rate should I use?
The S&P 500 has returned ~10% annualized historically. After inflation, the real return is around 7%. For conservative estimates, use 6–7%.