If you save ₩1,000,000 per month for 30 years, how much do you end up with? Sum it up and you get ₩360M. But if that money grows at 7% per year, the outcome is completely different. This post explains, in numbers, why "compound interest" keeps coming out of Warren Buffett's mouth.
Compound = "Interest on Interest"
Simple interest accrues on the principal only. Park ₩100M at 5% simple interest and you gain ₩5M every year — exactly. After 10 years, ₩150M. Linear.
Compound interest turns yesterday's interest into today's principal. The ₩5M earned in year 1 becomes part of the year-2 base, and the same 5% applies on top of it. Stretch this 30 years and the gap explodes.
Simple: 100M × (1 + 0.05 × 30) = 250M
Compound: 100M × (1 + 0.05)^30 ≈ 432M
Same principal, same rate, same horizon. The result differs by 1.7×.
Monthly DCA Compounding — Where the Magic Really Starts
Most people can't drop ₩100M in one go. But ₩1M every month — that's doable. What happens when each month's contribution starts compounding the moment it lands?
7% annual return, ₩1M monthly, 30 years. Total contributions: ₩360M. The compound result: about ₩1,220M (≈ ₩1.22B). Which means ₩860M is pure growth — more than 2× what you put in.
Cut just 5 years off — at 25 years, you end up around ₩800M. Five years cost ₩400M. This is the mathematical case for "start early."
Don't Forget Korean Taxes
In Korea, interest on bank deposits is automatically taxed at 15.4% (14% income + 1.4% local). Capital gains on overseas ETFs run 22% (with a ₩2.5M annual deduction), and US ETF dividend distributions get 15% withholding upfront. ₩1.22B "before tax" and "after tax" are not the same number.
Apply 15.4% tax to the simulation above and the ₩1.22B drops to about ₩1.09B — ₩130M evaporates as tax. This is exactly why tax-advantaged Korean accounts (ISA, 연금저축, IRP) are powerful for long-term investing.
Inflation — The Real Enemy
Will ₩1.22B in 30 years be worth ₩1.22B today? No. With 2.5% annual inflation, the purchasing power of money shrinks to about 47.7% of today's value over 30 years. That ₩1.22B in the future buys roughly ₩570M worth of stuff at today's prices.
This is why "real value" matters. Subtract inflation (2.5%) from nominal return (7%) and you get a real return of about 4.5%. That's the speed at which your wealth actually grows.
How to Get Started
- Time beats amount: ₩500K/mo for 30 years beats ₩2M/mo for 10 years.
- Tax-advantaged first: max out ISA / 연금저축 (Korean retirement) before going to a regular brokerage account.
- Index ETFs: KODEX200, S&P 500, ACE US S&P 500 — historical 7~10% expected return.
- Auto-debit: don't try to time the market. Same day, same amount, every month.
- Don't touch it: compounding lives on holding. Over 30 years you will see at least one −50% drawdown. Survive it.
Try the Numbers Yourself
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Numbers in this post are simulation estimates based on assumed inputs. Actual returns and taxes vary by market conditions, product type, and your personal tax bracket.