1. The Reality Check
Albert Einstein reportedly called compound interest "the eighth wonder of the world." Whether or not he actually said it, the sentiment holds: compound interest is the single most powerful force in personal finance, and it works for you when you save, and brutally against you when you borrow.
2. Plain English Definition
Simple interest pays you a fixed amount each period based only on your original deposit. Compound interest pays you interest on your original deposit plus all the interest you've already earned. Your money starts earning money on its own money.
3. Why It Snowballs
In year one, $1,000 at 10% earns $100. In year two, you earn 10% on $1,100, not $1,000 — that's $110. The gap seems tiny at first, but over 20-30 years this snowball effect is why early investors consistently beat late investors, even ones who contribute more money overall.
4. Key Terms
- Principal: Your original deposit or loan amount.
- Compounding Frequency: How often interest is calculated and added — annually, monthly, or daily. More frequent compounding means faster growth.
- APY: Annual Percentage Yield — the real return after compounding is factored in.
5. Next Steps
Use the calculator above to see exactly how your specific principal, rate, and timeline compounds — small changes in rate or time horizon make an outsized difference.
