Compound interest for savers: the only investment lesson most people need

Compound interest is the mechanism by which your investment returns themselves start generating returns. Over short horizons it is barely visible. Over long horizons it is the single most important factor in whether you build real wealth or not. Here is the one lesson, with the numbers that make it obvious.

The mechanic

You invest €10,000 at a 7% annual return. After year 1 you have €10,700. After year 2, you earn 7% on the €10,700 (not just the original €10,000) — you now have €11,449. The "extra" €49 is interest on last year's interest. Trivial in year 2. By year 20, the compounding effect dominates.

The numbers that make it obvious

€10,000 invested at 7% annual return for 10 years = €19,672. (Original doubles.)

Same money for 20 years = €38,697. (Tripled.)

Same money for 30 years = €76,123. (Multiplied by 7.6×.)

Same money for 40 years = €149,744. (Multiplied by 15×.)

The "doubling time" at 7% is about 10 years. Every decade of patience doubles the pile.

The small-contribution version

€200/month invested at 7% for 40 years = €525,000. The €96,000 you actually invested compounds to five times that figure. Starting at 25 vs 35 roughly doubles your end balance — the first decade of contribution is worth more than the last two combined.

The real-world adjustments

Inflation reduces purchasing power — 7% nominal is roughly 4-5% real in a 2-3% inflation environment. Taxes further reduce. Fees matter: a 1% annual expense ratio costs 20% of your terminal wealth over 30 years. Pick low-fee broad-market ETFs if you are not sure where to start.

The pattern most people miss

They see "7% a year" and think it means 7% × 30 = 210% total return. In reality it is 760% because of compounding. The intuition that "a small annual return cannot really matter" is wrong at long horizons.

The one implication

Start now. Not when you can "really afford it". Not when you have "a real amount to invest". €100/month at 7% for 40 years is still €260,000. The time horizon matters more than the amount.