What is compound interest?
Compound interest is interest calculated on both the initial principal AND the accumulated interest from previous periods. In other words, you earn interest on your interest.
Simple interest: $10,000 at 7% for 10 years = $17,000 (interest only on original $10,000)
Compound interest: $10,000 at 7% for 10 years = $19,672 (interest on growing balance)
The difference grows exponentially over time — this is why long-term investing is so powerful.
The compound interest formula
A = P(1 + r/n)^(nt)
Where:
- A = Final amount
- P = Principal (initial investment)
- r = Annual interest rate (decimal)
- n = Compounding frequency per year
- t = Time in years
Compounding frequency matters
$10,000 at 10% annual rate over 10 years:
| Frequency | Final Value | Interest Earned |
|---|---|---|
| Annually | $25,937 | $15,937 |
| Semi-annually | $26,533 | $16,533 |
| Quarterly | $26,851 | $16,851 |
| Monthly | $27,070 | $17,070 |
| Daily | $27,183 | $17,183 |
More frequent compounding = slightly more growth. For most investments, monthly compounding is standard.
The Rule of 72
A simple mental math shortcut: divide 72 by the interest rate to estimate how many years it takes to double your money.
| Rate | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
At 7% (approximately S&P 500 historical average after inflation), money doubles every ~10 years.
The power of time — why starting early matters
$10,000 invested at 7% annual return:
- After 10 years: $19,672
- After 20 years: $38,697
- After 30 years: $76,123
- After 40 years: $149,745
The second 20 years ($76,123 − $38,697 = $37,426) earns more than the first 20 years AND the original principal combined.
Monthly contributions make a huge difference
$10,000 initial + $500/month at 7% annual return:
| Years | Balance (no contributions) | Balance (with $500/month) |
|---|---|---|
| 10 | $19,672 | $106,765 |
| 20 | $38,697 | $281,573 |
| 30 | $76,123 | $594,234 |
Monthly contributions of just $500 (total $180,000 contributed) grew to $594,234 — returning $414,234 in compound interest.
Common investment vehicles and typical returns
| Investment | Average annual return | Risk level |
|---|---|---|
| Savings account | 2–5% | Very low |
| Government bonds | 3–6% | Low |
| Index funds (S&P 500) | 7–10% | Medium |
| Real estate | 7–11% | Medium |
| Individual stocks | Variable | High |
Past returns do not guarantee future performance.
Investing in Vietnam
Vietnamese investors can access compound growth through:
- Bank savings (tiết kiệm): 5–7% annual (Vietnamese Dong)
- Government bonds (trái phiếu chính phủ): 5–8% annual
- Stock market (VN-Index): historically 8–12% but volatile
- Real estate: significant growth in major cities
How to calculate compound interest free
- Go to Compound Interest Calculator
- Enter principal, monthly contribution, rate, and time period
- See year-by-year growth breakdown
- Compare compounding frequencies
- Use Rule of 72 calculator for quick mental math
Disclaimer: Investment returns are not guaranteed. Past performance does not predict future results. Consult a financial advisor before making investment decisions.