The Fundamentals
Compound interest glossary
| Term | Plain English Definition | Example |
|---|---|---|
| Compound interest | Interest on interest — your earnings generate their own earnings | $1,000 at 7% = $70 in Year 1, then 7% on $1,070 in Year 2 |
| Simple interest | Interest on principal only | $1,000 at 7% = $70 every year, always |
| Principal | Your starting investment amount | The $10,000 you put in initially |
| Compounding period | How often interest is applied | Daily, monthly, or annually |
| Future value | What your investment grows to | $10,000 becomes $76,123 at 7% over 30 years |
| Real return | Return after subtracting inflation | 7% nominal − 3% inflation = 4% real |
| APY | Annual Percentage Yield — true return including compounding | 5% APR compounded monthly = 5.12% APY |
Common Questions About How It Works
Q: How fast does compound interest double my money? Use the Rule of 72. Divide 72 by your annual rate. At 7%: 72÷7 = 10.3 years. At 10%: 7.2 years. At 3%: 24 years.
Q: Does compound interest work in a savings account? Yes — any savings account that adds interest to your balance, which then earns more interest, is compounding. The rate matters enormously. A 4.75% HYSA compounds your balance meaningfully; a 0.01% bank account barely compounds at all.
Q: What’s the best compounding frequency? Mathematically, daily beats monthly beats annually. Practically, the differences are tiny. On $10,000 at 5% over 20 years: daily = $27,126; monthly = $27,126 (essentially identical); annually = $26,533. Focus on rate and contributions, not compounding frequency.
Investment Account Questions
Q: Does compound interest work in a stock market investment? Stock market returns aren’t technically compound interest, but they behave identically when dividends are reinvested and gains are left to grow. The S&P 500's ~10% historical annual return, when reinvested over 30 years, produces the same exponential growth as compound interest.
Q: What is the difference between Roth and Traditional for compound interest? The compound growth rate is the same. The difference is tax treatment: Roth compounds tax-free forever; Traditional compounds tax-deferred (taxed at withdrawal). For most middle-income investors, the tax-free growth of Roth produces more after-tax wealth at retirement.
Quick-answer compound interest FAQ
| Question | Short Answer |
|---|---|
| Can compound interest make me rich? | Yes, with enough time and consistent contributions |
| Do I need to actively manage my investments? | No — index funds and automation handle it |
| Is compound interest guaranteed? | No — it depends on actual returns |
| What happens in negative years? | Your balance drops, but then compounds on the new lower base when returns resume |
| Should I reinvest dividends? | Yes — always. This is the compound interest mechanism in equity accounts |
| How do I know if I’m on track? | Run the calculator; compare to Fidelity benchmarks (1x salary at 30, 3x at 40) |
Tax and Account Limit Questions
Q: What are the 2025 contribution limits? 401(k): $23,500 ($31,000 if age 50+). IRA: $7,000 ($8,000 if age 50+). HSA: $4,300 individual, $8,550 family. Solo 401(k): up to $69,000. SEP-IRA: 25% of net income up to $69,000.
Q: What is the income limit for Roth IRA contributions? Phase-out begins at $150,000 for single filers and $236,000 for married filing jointly in 2025. Above these limits, use the backdoor Roth IRA strategy.
Put the Answers to Work
Enter your numbers in the compound interest calculator — see how every answer above applies specifically to your situation.