Compound Interest: What It Is and Why It Matters

Here's the difference in practice. With simple interest, you earn a return only on your original investment (the "principal"). With compound interest, you earn a return on your principal and on all the returns you've already accumulated. The longer you leave it, the faster it grows.

A simple example: Invest $10,000 at a 7% annual return. With simple interest, you'd earn $700 every year — $7,000 over 10 years. With compound interest, after 10 years you'd have approximately $19,672 — almost double your original investment, without adding a single extra dollar.

The key variable is time. The earlier you start investing, the more time compounding has to work. A 25-year-old who invests $5,000 and leaves it alone could end up with more wealth at 65 than a 40-year-old who invests $15,000 — purely because of the extra 15 years of compounding.

Compounding works in reverse too. Credit card debt at 20% annual interest compounds against you just as aggressively. This is why high-interest debt is the first thing to eliminate before building wealth.

The most powerful strategy to harness compounding is also the simplest: start early, invest consistently, and avoid withdrawing. Even small, regular contributions — $100/month — can grow into six figures over 30 years at average market returns.