1. What compound interest is
Compound interest is the phenomenon where the returns generated by an investment themselves start generating new returns. It is "interest on interest".
The base formula is simple:
FV = PV · (1 + r)^n
Where:
- FV = Future Value
- PV = Present Value invested
- r = Annual real rate of return (above inflation)
- n = Number of years
For example: €10,000 invested today, at 7% real over 20 years, becomes:
FV = 10,000 · (1 + 0.07)^20 = €38,697
Without doing anything else — just letting the money work.
2. Concrete example: €100/month for 30 years
Imagine you invest €100 per month in an MSCI World ETF with an estimated real return of 7% per year (above inflation) [⚠ to validate — MSCI World historical real return ~7-8%, no future guarantee].
After 30 years, your accumulated capital would be approximately €121,997 [⚠ to validate with exact calculator].
Of those €121,997:
- You contributed only €36,000 (€100 × 12 × 30)
- The remaining ~€86,000 came from compounded returns
In other words: more than two thirds of the final value cost nothing — it was time working.
3. Comparison: 10, 20 and 30 years at €100/month
| Years | Capital invested | Estimated final value | Returns generated |
|---|---|---|---|
| 10 years | €12,000 | ~€17,409 [⚠] | ~€5,409 |
| 20 years | €24,000 | ~€52,093 [⚠] | ~€28,093 |
| 30 years | €36,000 | ~€121,997 [⚠] | ~€85,997 |
The pattern is clear: compound interest accelerates exponentially with time. In the first 10 years it looks small. In the next 10 years the pace picks up. In the third decade, growth is explosive.
4. Why starting early matters so much
Consider two scenarios:
Ana starts investing €100/month at age 25 and stops at 35 (only invests for 10 years, total €12,000). She lets the money grow untouched until 65.
Bruno waits until 35 and invests €100/month for 30 years (invests 3x more, total €36,000).
Estimated result [⚠ to validate] at age 65 @ 7% real:
- Ana: invested €12,000 → ~€116,000
- Bruno: invested €36,000 → ~€122,000
Ana invested 3x less money and ended up almost at the same place. The difference was time.
The best time to start investing was 10 years ago. The second best time is today.
5. How to apply this
The most accessible ways to take advantage of compound interest:
- Accumulation ETFs (e.g. MSCI World, S&P 500 via a broker) — automatically reinvest dividends, maximising the compound effect
- Tax-efficient retirement accounts — vary by country, often offer tax deductions in exchange for lower liquidity
- High-yield savings + automatic transfers — lower returns but no market risk [⚠ current rates to validate]
The Goal Setter calculator lets you simulate how much you need to invest per month to reach your financial target, based on the same compound interest principles.
The numbers in this article are estimates for educational purposes. Past returns do not guarantee future returns. Consult a financial advisor authorised by CMVM before making investment decisions.