Compound Interest Explained: Why Einstein Called It the 8th Wonder of the World

Compound interest is earning interest on your interest — and it creates an exponential growth curve that feels almost magical over long time periods. It's also why debt is so dangerous: compound interest works against you just as powerfully when you're the borrower.

Simple vs Compound Interest

Simple interest: $10,000 at 7% for 30 years = $10,000 + ($700 × 30) = $31,000. Compound interest (annual): same numbers = $76,123. The difference — $45,000 — comes entirely from earning returns on your returns. Over longer periods, this gap becomes enormous.

The Rule of 72

Divide 72 by your annual interest rate to find how many years it takes to double your money. At 6%: 72 ÷ 6 = 12 years to double. At 9%: 8 years. At 12%: 6 years. This simple rule shows why higher returns and longer time horizons are so powerful.

Starting Early vs Investing More

Sarah invests $5,000/year from age 25 to 35 (10 years, $50,000 total), then stops. Tom invests $5,000/year from age 35 to 65 (30 years, $150,000 total). At age 65, assuming 7% returns: Sarah has ~$602,000. Tom has ~$472,000. Sarah wins — despite investing $100,000 less — because she started earlier.

How to Maximize Compound Interest

• Start investing as early as possible — time is the biggest variable
• Reinvest all dividends automatically
• Minimize fees — a 1% fee costs you ~25% of your final wealth over 30 years
• Avoid withdrawing early — every withdrawal resets the compounding clock
• Increase contributions over time as income grows