How Does Compound Interest Work?
A clear explanation of compound interest, why it accelerates over time, and how compounding frequency changes the result.
Compound interest is often summarized as "interest on interest," which is accurate but doesn't quite convey why it matters so much. The real story is about acceleration: growth that starts slow and keeps speeding up, for as long as the money stays invested.
Simple Interest vs. Compound Interest
Simple interest is calculated only on the original amount you deposited. If you put $1,000 in an account earning 5% simple interest, you'd earn $50 every year, forever — a flat, unchanging amount. Compound interest is calculated on your current balance, which includes any interest already added. That same $1,000 at 5% compounded annually earns $50 in year one, bringing the balance to $1,050. In year two, the 5% applies to $1,050, not $1,000 — earning $52.50 instead. The gap seems tiny at first, but it widens every year.
Why the Effect Accelerates Over Time
Because each period's interest is added to a slightly larger base than the one before, the dollar amount of interest earned grows every period too — even though the rate never changes. Over a few years the difference from simple interest is minor. Over a few decades, it's dramatic. $10,000 at 7% simple interest over 30 years grows to $31,000. The same $10,000 at 7% compounded annually grows to about $76,000 — more than double, purely from interest building on interest.
Compounding Frequency Matters, But Less Than Time Does
Interest can compound annually, monthly, daily, or on almost any schedule a financial product defines. More frequent compounding produces a slightly higher return at the same stated annual rate, because interest starts earning its own interest sooner. The effect is real, but it's modest compared to the two variables that matter far more: the rate itself, and — above all — how long the money stays invested.
The Practical Takeaway
Compound interest rewards time more than almost anything else. Starting early, even with a small amount, often outperforms starting later with a much larger sum, simply because the earlier money has had more compounding periods to work through.
Try the numbers for your own situation with the compound interest calculator, or see how compounding plays out inside a full portfolio projection with the investment calculator.