Investing
Compound Interest: The Eighth Wonder — Real Numbers Demonstration
Gary Chung（FinKit Editor-in-Chief） · Published：June 6, 2026
Einstein reportedly called compound interest the "eighth wonder of the world." This article uses real numbers to show just how much difference starting 10 years earlier can make.
What Is Compound Interest?
Compound interest means earning "interest on interest." Not only does your principal earn returns — the returns you've already accumulated also earn returns. The longer the time horizon, the more dramatic the compounding effect.
Real Numbers Demonstration
Assume you save HK$5,000 per month with an annual return of 7% (close to the long-term average of US equities):
| Saving Period | Total Invested | After Compounding | Returns |
|---|---|---|---|
| 10 years | HK$600K | HK$870K | HK$270K |
| 20 years | HK$1.2M | HK$2.62M | HK$1.42M |
| 30 years | HK$1.8M | HK$6.13M | HK$4.33M |
| 40 years | HK$2.4M | HK$13.2M | HK$10.8M |
The Power of Starting 10 Years Earlier
Alex starts saving HK$5,000/month at age 25 and stops at 35 (HK$600K total invested). He never adds another dollar — just lets compounding work until age 65:
Final amount: ~HK$6.63M
Brian starts at age 35 and saves HK$5,000/month all the way to 65 (HK$1.8M total invested):
Final amount: ~HK$6.13M
Alex invested two-thirds less — yet ended up with more than Brian. That is the power of compound interest plus time.
The Rule of 72: Quick Estimation
Want to know how long it takes for your money to double? Use the Rule of 72: 72 ÷ annual return rate = years to double
- 7% return → 72 ÷ 7 ≈ 10.3 years to double
- 10% return → 72 ÷ 10 = 7.2 years to double
- 4% return → 72 ÷ 4 = 18 years to double
Try it yourself:
Compound Interest Calculator →