Quick explanation
Compound interest is what happens when your earnings start earning on their own. Instead of collecting interest only on the money you originally deposited, you earn interest on the interest itself. Over short periods this barely matters, but over decades the effect is dramatic — a single deposit can multiply many times over without you adding another cent. The concept applies far beyond savings accounts: credit card debt compounds against you, investment returns compound in your favor, and even inflation compounds to erode purchasing power. Understanding this one idea changes how you think about saving early, paying down debt, and evaluating any financial product that quotes an annual rate. Compound interest is the engine behind retirement funds, mortgage amortization tables, and the reason financial advisors repeat the phrase "start now." It is arguably the most important concept in personal finance, yet most people encounter it only as a formula in a textbook rather than as an intuition they carry into real decisions.
What you'll learn
- 1How compound interest differs from simple interest
- 2Why time matters more than the amount you invest
- 3The Rule of 72 for quick mental math on doubling time
- 4How compounding frequency affects growth
- 5Why compound interest works against you with debt
Sample Whet lesson preview
“If you invested $1,000 at age 20 and never added another dollar, it could grow to over $88,000 by retirement at a 10% average annual return.”
Simple vs. compound interest
Simple interest pays you only on your original deposit. If you put $1,000 in a 5% simple-interest account, you earn $50 every year — forever $50. Compound interest pays you on the growing total. After year one you have $1,050; in year two you earn 5% on $1,050, giving you $1,102.50. The gap starts small but widens every year because each cycle's base is larger than the last.
You invest $1,000 at 5% compounded annually. After 2 years, roughly how much do you have?
- A$1,100.00
- B$1,102.50
- C$1,105.00
- D$1,050.00
Key takeaways
- Compound interest earns returns on previous returns, creating exponential growth
- Starting 10 years earlier can matter more than doubling your monthly contribution
- The Rule of 72 lets you estimate doubling time by dividing 72 by the annual rate
- Debt compounds too — high-interest credit cards use the same math in reverse
Why learn this with Whet
Whet breaks compound interest into a short, focused lesson with a hook that grabs your attention, clear explanations you can read in a few minutes, and a quiz that checks whether you actually understood — not just whether you read the words. After the lesson, Whet schedules spaced-repetition reviews so the concept sticks weeks and months later, not just the afternoon you learned it. The lesson connects to your growing knowledge graph, linking compound interest to related ideas like inflation, the time value of money, and risk diversification so you build a web of understanding rather than isolated facts. Five minutes now, retained for good.
Frequently asked questions
- Not at all. The core idea is simple: your earnings earn their own earnings. You can grasp the concept with basic multiplication. The formulas exist for precision, but the intuition — that time amplifies small amounts — is what matters for everyday financial decisions. Whet focuses on building that intuition rather than drilling equations.
- Compound interest is a mechanism, not an investment type. Savings accounts, bonds, and even stock market index funds all use compounding — your returns generate further returns. The difference between vehicles is the rate and the risk. A savings account compounds at a low, guaranteed rate; stocks compound at a higher average rate but with volatility along the way.
- Because compounding needs time to work. An extra decade of compounding can easily double or triple the final amount, even if you never increase your contributions. Someone who invests $200 a month from age 25 to 35 and then stops can end up with more than someone who invests $200 a month from age 35 to 65, simply because the early investor gave their money more time to compound.
- Yes, and that is exactly why high-interest debt is so dangerous. Credit card balances typically compound daily at rates above 20% APR. If you carry a balance, you pay interest on yesterday's interest. The same force that grows your savings works against you with debt, which is why financial advisors prioritize paying off high-interest debt before investing.