Compound Interest: A Complete Guide — How Interest on Interest Creates a Snowball Effect
Simple vs. Compound Interest — The 20-year Gap
What does ₩10M invested at 7% for 20 years become?
- Simple interest: ₩10M + (₩10M × 7% × 20 years) = ₩24M
- Compound interest: ₩10M × (1.07)²⁰ = ₩38.7M
Same principal, same rate, same period — yet compound interest produces ₩14.7M more. That’s the essence of compounding: the interest you earn becomes part of your principal, generating further interest in turn. Interest earns interest.
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Compound Interest
Exponential Growth Dynamics
Experience the magic where interest earns interest. Graph the snowball effect of your wealth.
20년 뒤 예상 자산
₩561,314,048
Total Principal
₩250,000,000
Total Interest
+₩311,314,048
The Rule of 72 — Time to Double
A quick rule for estimating how long it takes compound interest to double your money:
72 ÷ Annual Return = Years to Double
- 6% annual return → 72 ÷ 6 = 12 years
- 8% annual return → 72 ÷ 8 = 9 years
- 12% annual return → 72 ÷ 12 = 6 years
The rule works in reverse too. If inflation is 4% per year, your cash’s purchasing power halves in 72 ÷ 4 = 18 years.
Why Time Beats Rate of Return
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Look at the difference between 30 and 40 years. At 10%, adding just 10 more years multiplies the final value by 2.8×. This is the mathematical proof behind “the earlier you start, the better.”
Improving your annual return from 5% to 7% (a 2 percentage-point gain) matters less than extending your investment horizon from 20 years to 30 years (an extra decade). In compound interest, time is the most powerful variable of all.
Practical Principles for Compound Investing
1. Never Stop — That’s the Best Strategy
Compound interest’s greatest enemy is withdrawing money partway through. Selling during a downturn or pausing contributions breaks the compounding chain. Use automatic transfers to link income directly to investing — remove willpower from the equation.
2. Minimize Costs
The impact of a 0.5% fee difference over 30 years is far larger than you’d expect. Fees also compound — but they compound against you, draining your returns every single year.
3. Use Tax Deferral
Tax-deferred accounts (pensions, ISAs, retirement savings plans, etc.) are compounding amplifiers. Money that would have gone to tax instead stays invested for decades — generating returns on those “saved” tax dollars year after year.
OIYO Editorial
Content Editor지식 인큐베이터이자 전문 콘텐츠 크리에이터. 경영, 경제, 법률 및 실생활에 유용한 실무/자격증 중심의 깊이 있는 정보를 연구하고 공유합니다.