The Complete Guide to Compound Interest: The Rule of 72, Inflation Adjustment & Growth Simulations
Introduction: What Einstein Called “The Eighth Wonder of the World”
“Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn’t, pays it.” — Attributed to Albert Einstein (precise origin uncertain, but the concept is mathematically exact)
Compare simple interest and compound interest in actual numbers and the quote makes perfect sense. Invest ₩10M at 7% for 30 years:
- Simple interest: ₩10M × 7% × 30 = ₩21M in interest → ₩31M total
- Compound interest: ₩10M × (1.07)³⁰ = ₩76.1M total
Compound interest produces 2.5× more than simple interest. The gap grows exponentially as the time horizon lengthens and the rate rises.
1. Key Compound Interest Numbers
2. Simple Interest vs. Compound Interest — The Growing Gap
| 구분 | Simple Interest | Compound Interest |
|---|---|---|
| After 5 years | ₩13.5M | ₩14.0M |
| After 10 years | ₩17.0M | ₩19.7M |
| After 15 years | ₩20.5M | ₩27.6M |
| After 20 years | ₩24.0M | ₩38.7M |
| After 25 years | ₩27.5M | ₩54.3M |
| After 30 years | ₩31.0M | ₩76.1M |
| After 40 years | ₩38.0M | ₩149.7M |
Compound interest shines in the back half
The compound curve looks almost identical to simple interest in the early years. At 10 years: simple ₩17M vs. compound ₩19.7M — a ₩2.7M difference. But at 40 years: simple ₩38M vs. compound ₩150M — a 4× gap.
This is the mathematical foundation of the advice “starting early matters more than investing more.”
3. The Rule of 72: Mental Math in 2 Seconds
The Rule of 72 is a quick mental calculation for how long it takes for compound interest to double your principal.
Years to double ≈ 72 ÷ Annual Return (%)
Years to Double Principal by Return Rate (Rule of 72)
Reverse-applying the Rule of 72:
- Want to double in 10 years? 72 ÷ 10 = you need at least a 7.2% return
- Want to double in 20 years? 72 ÷ 20 = 3.6% — achievable with high-yield deposits
4. Monthly Contribution Simulation
More realistic than a lump sum is investing a fixed amount every month.
Final Portfolio Value: ₩300K/month Contributions (7% p.a. compound)
Units: ₩10,000. ₩300K/month × 30 years = ₩108M contributed → compound effect takes it to ₩366M
Final Value by Monthly Contribution Amount (30 years, 7% p.a.)
Units: ₩10,000
5. Inflation-adjusted Real Returns
Nominal returns alone are misleading. You need to subtract inflation to see your actual increase in purchasing power.
Real Return ≈ Nominal Return − Inflation Rate (More precisely: Real Return = (1 + Nominal) ÷ (1 + Inflation) − 1)
| 구분 | Asset Type (Nominal) | Real Return After 2.5% Inflation |
|---|---|---|
| Demand deposit 1.5% | +1.5% | −1.0% (real loss) |
| 1-year term deposit 3.5% | +3.5% | +1.0% |
| Government bonds 4.0% | +4.0% | +1.5% |
| REITs 6.0% | +6.0% | +3.5% |
| S&P 500 ETF 10.7% | +10.7% | +8.2% |
| Growth portfolio 14% | +14.0% | +11.5% |
Why keeping money in a savings account can make you poorer
When bank interest is lower than inflation, the number in your account grows but your purchasing power shrinks. If inflation runs at 3–4% while your deposit earns 3.5%, your real return is between −0.5% and +0.5%.
Over the long term, preserving purchasing power requires compound investment in assets that outpace inflation.
6. Compound Investing Action Timeline
7. The Biggest Enemies of Compound Growth
| 구분 | Compound Killers | Compound Boosters |
|---|---|---|
| Fees | A 1.5% annual fund fee erodes 30%+ of your final portfolio over 30 years. Passive ETFs (0.03–0.2%) beat active funds in most long-term comparisons | Low-cost index ETFs (e.g., Vanguard, iShares, Fidelity zero-fee funds) |
| Tax | Dividend and interest taxes, capital gains tax, and frequent trading in taxable accounts all reduce compounding | Max out tax-advantaged accounts (ISA, pension, IRA equivalents) for tax-free or tax-deferred compounding |
| Market Timing | Trying to pick tops and bottoms. Investors who attempt this typically underperform the market by 1.5–3% per year | Dollar-cost averaging (DCA). Let time work for you rather than trying to outsmart the market |
| Psychology | Panic-selling in a crash → selling low and buying back high in a cycle | Automate investments and resist checking your portfolio obsessively. Tolerating boredom is a strategy |
| Early Withdrawal | Cashing out a pension or investment account early — principal loss, taxes, penalties, and broken compounding | Separate accounts by purpose — emergency fund apart from investments; never touch investment capital |
Closing Thought: Compound Interest Is Patience, Not Skill
Did you know Warren Buffett accumulated 99% of his net worth after the age of 50? He bought his first stock at 11 and never stopped compounding for more than 60 years. The power of compound interest comes not from genius stock-picking but from time × consistency.
Calculate it for yourself today.
Further Reading
- Dividend Investing Guide — Selecting Dividend Stocks, Calculating Yield
- Inflation and Purchasing Power — Protecting the Real Value of Your Money
References
- Bogle, J. C. (2007). The Little Book of Common Sense Investing. Wiley.
- Bernstein, W. J. (2002). The Four Pillars of Investing. McGraw-Hill.
- S&P 500 Historical Returns — Macrotrends
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