"Compound interest is the 8th wonder of the world. He who understands it, earns it; he who doesn't, pays it." This concept applies not just to savings accounts but to staking yields, DeFi APY, and reinvested trading profits — and understanding it changes how you think about every investment decision.
Simple interest earns returns only on the principal. If you invest $1,000 at 10% annually for 10 years, you earn $100/year = $2,000 total ($1,000 profit).
Compound interest earns returns on both principal AND accumulated interest. That same $1,000 at 10% compounding annually grows to $2,593 after 10 years — 159% more.
The Rule of 72 is a quick mental shortcut: 72 ÷ interest rate = years to double your money.
| Annual Return | Years to Double | Example |
|---|---|---|
| 5% (savings) | 14.4 years | High-yield savings account |
| 10% (S&P 500 hist.) | 7.2 years | Index fund |
| 20% | 3.6 years | Strong stock/crypto year |
| 72% | 1 year | Exceptional crypto bull run |
Starting early beats starting smart. Consider two investors both earning 10% annually:
Alice invests the same amount but earns 2.6× more — purely because of 10 extra years of compounding.
Crypto introduces compounding at rates traditional finance never sees — for better or worse:
A 100% APY sounds incredible — but if the yield is paid in a governance token that loses 80% of its value, your real return may be negative. Always calculate APY in terms of a stable unit (USD or BTC). Focus on "real yield" — protocols that pay fees in ETH, USDC, or other established assets.