The Eighth Wonder of the World
Compound interest is often called the most powerful force in finance. The concept is simple: you earn interest not just on your original investment, but also on the interest you have already earned. Over time, this creates exponential growth that can turn modest savings into substantial wealth.
Investor B starts at age 35, invests $300/month for 30 years (age 35-65). Total invested: $108,000.
At 8% annual returns:
Investor A at age 65: ~$510,000 (invested $36K)
Investor B at age 65: ~$440,000 (invested $108K)
Investor A invested 3x less money but ended up with more because those early dollars had 40 years to compound.
Run your own compound interest scenarios.
Open Compound Interest Calculator →The Rule of 72
A quick way to estimate how long it takes to double your money: divide 72 by your annual return rate. At 8% returns, your money doubles every 9 years (72 ÷ 8 = 9). At 10%, every 7.2 years.
| Return Rate | Doubles Every | $10K Becomes in 30 Years |
|---|---|---|
| 6% | 12 years | $57,400 |
| 8% | 9 years | $100,600 |
| 10% | 7.2 years | $174,500 |
| 12% | 6 years | $299,600 |
The Three Variables You Control
Compound interest has three inputs: how much you invest, what return you earn, and how long you let it grow. Of these three, time is the most powerful because it is the exponent in the formula. You cannot control the market, but you can control when you start.
The best time to start investing was 10 years ago. The second best time is today.