Compound interest is the foundation of all long-term investing. Your money earns returns. Those returns earn their own returns. Over decades, the growth becomes exponential — and the numbers get genuinely surprising.
The formula that builds wealth
The compound interest formula is straightforward: A = P(1 + r/n)^(nt), where P is your principal, r is the annual rate, n is the number of compounding periods per year, and t is the number of years. But the real power comes when you add regular contributions on top.
With monthly contributions, the math becomes even more interesting. Each monthly deposit starts compounding from the moment it enters your account. The earlier deposits have more time to grow, creating a snowball effect.
Total contributed: €72,000
Final portfolio value: €244,000
Pure growth (free money): €172,000
Your money more than tripled without any effort.
Why the rate matters more than the amount
Most people focus on how much they can save each month. Important, yes — but the return rate has a far bigger impact over long periods. Consider €300/month over 30 years:
| Annual return | Portfolio after 30y | Your contribution | Free growth |
|---|---|---|---|
| 3% (savings account) | €175,000 | €108,000 | €67,000 |
| 5% (balanced portfolio) | €250,000 | €108,000 | €142,000 |
| 7% (stock-heavy portfolio) | €366,000 | €108,000 | €258,000 |
| 9% (aggressive growth) | €542,000 | €108,000 | €434,000 |
Same contribution. Same time period. The 9% scenario gives you 3× more than the 3% scenario. This is why keeping investment fees low matters — a 1% annual fee on a 8% portfolio reduces it to 7%, which costs you €176,000 over 30 years.
The cost of waiting
Time is the one resource you can't get back. Every year you delay starting, the final number shrinks dramatically. Here's what €200/month at 7% looks like depending on when you start:
| Investing horizon | Portfolio value | Lost vs. starting 10y earlier |
|---|---|---|
| 40 years (start at 25) | €528,000 | — |
| 30 years (start at 35) | €244,000 | €284,000 lost |
| 20 years (start at 45) | €104,000 | €424,000 lost |
| 10 years (start at 55) | €34,600 | €493,400 lost |
Starting at 25 vs. 55 is the difference between €528,000 and €34,600. Same monthly amount, same return rate. The only difference is time. This is why financial advisors always say: start now, not later.
Inflation: the silent destroyer
Here's what most compound interest calculators miss: inflation. At 2.5% annual inflation, your €244,000 after 30 years has the purchasing power of about €118,000 in today's money. The nominal number looks great, but you need to think in real terms.
This is why the WealthRank compound interest calculator shows both nominal and inflation-adjusted values. The "real" return of a 7% portfolio after 2.5% inflation is about 4.5% — still positive, still powerful, but the headline number needs context.
What return rate should you use?
Based on historical data from the MSCI World Index and S&P 500:
| Portfolio type | Historical real return | Use for |
|---|---|---|
| 100% equities (global) | ~7% nominal / ~4.5% real | Optimistic projection |
| 80/20 stocks/bonds | ~6% nominal / ~3.5% real | Most common scenario |
| 60/40 stocks/bonds | ~5% nominal / ~2.5% real | Conservative estimate |
| 100% bonds | ~3% nominal / ~0.5% real | Cautious / near retirement |
Historical returns don't guarantee future performance, but they're the best benchmark we have. For most people under 40, a stock-heavy portfolio is appropriate. Use the compound interest calculator to model your own scenarios — and don't forget to factor in fees.
Key takeaways
1. Start now. Time is the most important variable. Even small amounts matter if you give them enough years.
2. Focus on fees. A 1% fee difference costs hundreds of thousands over a career. Low-cost index funds are your friend.
3. Think in real terms. Always subtract inflation from your expected return to understand actual purchasing power.
4. Be consistent. Regular monthly contributions, automated, remove emotion from the process.