What is compound interest and how does it grow your money?
Compound interest is one of the most powerful concepts in personal finance — and also one of the most underestimated. Unlike simple interest, where you only earn returns on your original capital, compound interest generates returns on both your principal and the interest already accumulated. The result is exponential growth rather than linear growth.
- Compound interest earns returns on both principal and accumulated interest
- Rule of 72: divide 72 by annual return to estimate years to double
- Starting 5 years earlier can mean 40% more final capital
- Regular monthly contributions dramatically amplify compound growth
- Always account for inflation to see real purchasing power
Simple interest vs. compound interest
With simple interest, you earn a fixed percentage of your original investment each year. If you invest €10,000 at 6% simple interest for 20 years, you earn €600 per year — €12,000 total — giving you €22,000. With compound interest at the same 6% rate, your €10,000 becomes €32,071 after 20 years. The extra €10,071 comes entirely from returns on returns. The longer the time horizon, the wider the gap becomes.
The formula explained simply
The compound interest formula is: Final Value = P × (1 + r/n)^(n×t). Where P is principal, r is the annual rate (as a decimal), n is compounding periods per year, and t is years. Investing €5,000 at 7% compounded monthly for 10 years: Final Value = 5,000 × (1.005833)^120 ≈ €10,048. Your money roughly doubled in 10 years.
The Rule of 72
Divide 72 by the annual return percentage to estimate years to double your money. At 6%: 72 ÷ 6 = 12 years. At 8%: 9 years. At 4%: 18 years. This shortcut helps you compare investment options and understand the long-term impact of seemingly small differences in return rates.
Why regular contributions amplify the effect
Adding €200/month to an initial €5,000 at 7% over 20 years results in approximately €109,000 — compared to just €19,348 with no contributions. The key insight: starting earlier matters far more than investing larger amounts later.
Inflation: the hidden factor
A 7% nominal return with 3% inflation gives a real return of only about 4%. Over 20 years, the difference is substantial. The FinSimLab compound interest calculator lets you apply an inflation adjustment to see projections in today's purchasing power.
Frequently asked questions
What is compound interest?
Earning returns on your previous returns, not just on the money you put in. Over long periods, the growth accelerates.
What is the Rule of 72?
Divide 72 by the annual return to estimate how many years it takes to double your money. At 6%, about 12 years.
Does compound interest work with small amounts?
Yes. Time matters more than the starting amount: small regular contributions over decades can grow substantially.
How does inflation affect compound interest?
It reduces the purchasing power of the result. Always look at the real return: nominal return minus inflation.
Read further
- The Little Book of Common Sense Investing (John C. Bogle). It gives you: The most direct explanation of why costs decide much of a fund's return.
- The Psychology of Money (Morgan Housel). It gives you: Explains like few others why behaviour matters more than knowledge when managing money.
- Just Keep Buying (Nick Maggiulli). It gives you: Puts data behind the questions other books answer with opinions: how much to save and when to buy.