Compound Interest Explained: The 8th Wonder of the World
Compound interest is the mechanism by which your returns generate their own returns. Over 20-30 years, it turns modest monthly investments into significant wealth — without any extra effort on your part.
Simple vs Compound Interest
Simple interest: you earn interest only on your initial capital. €10,000 at 7%/year = +€700/year, every year. After 30 years: €31,000.
Compound interest: you earn interest on your capital AND on the interest already earned. After year 1, your €700 of interest starts earning interest too. After 30 years at 7%: €76,100.
Same rate. Same initial capital. Same time. The difference: €45,100.
| Year | Simple interest (7%) | Compound interest (7%) |
|---|---|---|
| 1 | €10,700 | €10,700 |
| 5 | €13,500 | €14,026 |
| 10 | €17,000 | €19,672 |
| 20 | €24,000 | €38,697 |
| 30 | €31,000 | €76,123 |
The Curve Gets Steeper Over Time
Compound interest is slow at first — then explosive. That's why most people underestimate it: they give up too early, when the curve is still flat.
With €10,000 at 7%:
- Years 1-10: gained €9,672
- Years 10-20: gained €19,025 (almost 2x more)
- Years 20-30: gained €37,426 (almost 4x more than the first decade)
The last 10 years produce more wealth than the first 20. Time is the most powerful input in the equation.
Divide 72 by your annual return to find how many years until your capital doubles.
At 4% → doubles every 18 years.
At 7% → doubles every 10 years.
At 10% → doubles every 7 years.
At 7%, a €10,000 investment becomes €80,000 in 30 years — it doubles three times.
Compound Interest + Monthly Contributions
The real power comes when you add regular monthly contributions to compound interest. Each monthly payment starts its own compounding cycle.
| Scenario | Total invested | Value at 7% after 30 years |
|---|---|---|
| €10,000 lump sum only | €10,000 | €76,100 |
| €100/month only | €36,000 | €121,900 |
| €10,000 + €100/month | €46,000 | €198,000 |
| €200/month only | €72,000 | €243,800 |
Why Most People Don't Benefit From It
Three reasons compound interest doesn't work for most people:
- They start too late: at 40 instead of 25. Starting 15 years late costs more than doubling your monthly contributions can compensate for.
- They break the cycle: they sell during crashes, withdraw for non-emergency expenses, or stop DCA when life gets complicated.
- They choose the wrong vehicle: savings accounts at 2-3% barely beat inflation. The compound effect is tiny. Stocks at 7% average is what makes the math impressive.
How to Maximise Compound Interest
- Start as early as possible: today beats tomorrow
- Use accumulating ETFs: dividends automatically reinvested, no friction, no tax drag in PEA
- Never interrupt the cycle: keep DCA running through downturns
- Minimise fees: 1% extra in fees over 30 years reduces your final amount by ~25%
- Increase contributions over time: as income grows, scale up monthly amounts
See your compound growth in real time
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Try it for free →Frequently Asked Questions
Does compound interest work in a savings account?
Yes, but at 2-3%, the effect is minimal. At 3% over 30 years, €10,000 becomes €24,300. At 7% in the stock market, the same €10,000 becomes €76,100. The compounding formula is the same — the rate makes all the difference.
What is the best account for compound interest in France?
A PEA with accumulating ETFs. The PEA delays taxation until withdrawal (after 5 years at 18.6%), meaning dividends reinvest without annual tax drag. This is crucial for long-term compounding — every euro you'd have paid in tax keeps working for you instead.
Does compound interest work the same way in a bear market?
Compound interest is a mathematical property, not a market guarantee. In a prolonged bear market, your portfolio doesn't compound positively. But historically, the stock market has always recovered — and the years after a crash produce some of the best returns, which turbocharged compounding for those who stayed invested.