What is compound interest?

Compound interest is a notion of financial mathematics well known to investors and bankers. Compound interest is when interest earned on a principal in a given period is reinvested with that principal in the next period to earn more interest. We are talking about compound interest as opposed to "simple" interest that is not reinvested.

How compound interest works

The difference between simple interest and compound interest

To understand the power of compound interest, it is first necessary to distinguish it from simple interest. With simple interest, only your initial capital generates returns. If you invest 1.000€ at 10% simple interest for 10 years, you will receive 100€ each year, for a total gain of 1.000€.

Compound interest works differently: the interest earned is added to the capital and itself generates interest. The resulting wealth accumulation is not linear (the same amount of interest received each period) but exponential (a larger amount of interest with each period). This phenomenon of exponential accumulation therefore works in favor of the investor who benefits from it.

How do you calculate compound interest?

The value at time t of capital subject to compound interest is calculated using the following formula:

Capital at time t = Initial capital × (1 + interest rate) × (1 + interest rate) × … × (1 + interest rate)
= Initial capital × (1 + interest rate)^t

A concrete example: the magic of compound interest in action

An investment of 1.000€ over 10 years

To illustrate the power of compound interest in concrete terms, let’s consider a person who invests €1,000 at an annual interest rate of 10%. Here is the detailed evolution of their investment:

• Year 1: 1.000€ × 1.10 = 1.100€ (gain of 100€)
• Year 2: 1.100€ × 1.10 = 1.210€ (gain of 110€)
• Year 3: 1.210€ × 1.10 = 1.331€ (gain of 121€)
• Year 5: 1.610€ (total gain of 610€)
• Year 10: 2.594€ (total gain of 1.594€)

After 10 years, this person will not have generated a profit of 100% (i.e. 10 times 10%), but a profit of 159,4%, thanks to the reinvestment of the interest earned each year, which itself generated compound interest.

The impact of time on compound interest

The previous example reveals a fundamental truth: the longer the investment period, the more spectacular the effect of compound interest becomes. If our investor held their investment for 20 years instead of 10, their capital would reach 6.727€, representing a gain of 572%. This geometric progression explains why Warren Buffett recommends investing as early as possible and holding positions for the long term.

The key factors that maximize compound interest

The rate of return: every percentage point counts

The rate of return is the fuel of compound interest. An apparently small difference can have major long-term consequences. Let’s revisit our example of 1.000€ over 20 years:

• At 8%: 4.661€
• At 10%: 6.727€
• At 12%: 9.646€

This sensitivity to the rate of return explains the importance of selecting ETFs or index funds with low fees, as every percentage point saved turns into substantial gains through compound interest.

Easyvest puts compound interest to work for you

At Easyvest, our entire approach is based on the simplicity and discipline required to fully benefit from compound interest. By offering diversified ETF portfolios with low fees, automatically rebalanced and designed for the long term, we maximize your chances of benefiting from this snowball effect. Our mission is to make your money work for you, with transparency and rigor, so that patience and time become your greatest allies in building solid wealth.

Pitfalls to avoid in order to optimize compound interest

1. The impact of management fees

Management fees are the sworn enemy of compound interest. A fund with 2% annual fees versus an ETF at 0,2% can reduce your final capital by 30% over 30 years. This difference, amplified by compound interest, justifies a preference for passive management and low-cost products.

2. The temptation of early withdrawals

Interrupting the compounding process breaks the magic of compound interest. Every euro withdrawn prematurely deprives the investor of all the future gains that amount would have generated. This reality highlights the importance of building a separate emergency fund before investing.

FAQ: everything you need to know about compound interest

1. From what amount does compound interest become worthwhile?

Compound interest works regardless of the initial amount. Even 50€ per month, invested over 30 years at 8%, generates 75.000€ thanks to this mechanism. What matters is not the starting amount but the regularity and duration of the investment.

2. How long does it take to see the effects of compound interest?

The effect generally becomes visible after 7–10 years, a period when compound interest begins to represent a significant share of total gains.

3. Does compound interest apply to ETFs?

Absolutely. Accumulating ETFs automatically reinvest dividends, creating a continuous compounding effect. Distributing ETFs also allow investors to benefit from compound interest by manually reinvesting the distributions received.