How compound interest works — and why time is your greatest asset
Compound interest is the most powerful force in personal finance. Learn how it works, the formula behind it, and how to use it to build wealth over time.
What is compound interest?
Compound interest is interest calculated not just on your initial principal, but also on the interest you’ve already earned. In other words, you earn interest on your interest — and this effect accelerates dramatically over time.
Simple example:
- Deposit €1,000 at 8% annual interest.
- After year 1: 1,000 × 1.08 = €1,080 (+€80)
- After year 2: 1,080 × 1.08 = €1,166 (+€86, not €80)
- After year 10: €2,159 — balance doubled with no extra contributions.
- After year 30: €10,063 — a 10× multiple from interest alone!
Each year’s gain grows larger because the base is growing — this is the power of exponential growth.
The formula
For a lump sum with no contributions:
A = P × (1 + r/n)^(n × t)Where:
- A — the final balance
- P — the principal (initial investment)
- r — the annual interest rate (as a decimal, e.g. 0.08 for 8%)
- n — compounding periods per year (12 = monthly, 365 = daily)
- t — years
When you add regular contributions, each contribution compounds for the remaining time — our calculator models this directly.
The Rule of 72
Divide 72 by your interest rate to get the approximate number of years to double your money.
| Rate | Years to double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 12% | 6 years |
This rule is useful for quick estimates — and makes it clear why the interest rate matters so much.
What matters most?
Three factors directly drive compounding:
- Time — the single most powerful lever. Starting 10 years earlier often beats contributing far more money later.
- Interest rate — every extra basis point multiplies exponentially over long periods.
- Regular contributions — even small monthly amounts (€50–€100) build substantial wealth over decades.
The most expensive mistake: delay
Imagine two people starting from €0 and contributing €200/month at 7%:
- Anna starts at 25, stops at 65 → €528,000
- Ben starts at 35, stops at 65 → €243,000
Ben contributed €24,000 less — but Anna’s final balance is more than double. Ten years of head start made all the difference.
Calculate your own scenario with the Compound Interest Calculator.
How to maximise it
- Reinvest dividends automatically — don’t withdraw them.
- Add consistent monthly contributions, even if they’re small.
- Minimise interference — frequent buying and selling reduces real returns.
- Watch costs — a 1% management fee can cost tens of thousands over 30 years.