The Concept in One Sentence
Compound interest means you earn interest not just on your original money, but also on the interest you've already earned — making your money grow at an accelerating rate over time.
Simple Interest vs. Compound Interest
To understand compound interest, it helps to contrast it with simple interest.
Simple interest is calculated only on the original amount (called the principal). If you invest $1,000 at 10% simple interest per year, you earn $100 every year. After 10 years, you have $2,000.
Compound interest is calculated on the principal plus any interest already earned. At 10% compound interest:
- Year 1: $1,000 → $1,100
- Year 2: $1,100 → $1,210
- Year 3: $1,210 → $1,331
- Year 10: approximately $2,594
Same starting amount, same interest rate — but compound interest produces nearly $600 more after 10 years, and the gap widens dramatically over longer periods.
How Compounding Frequency Works
Interest can compound at different intervals: annually, monthly, daily, or even continuously. The more frequently it compounds, the faster your money grows.
| Compounding Frequency | $10,000 at 5% after 20 years |
|---|---|
| Annually | ~$26,533 |
| Monthly | ~$27,126 |
| Daily | ~$27,183 |
The differences between monthly and daily compounding are relatively small — but the jump from simple to compound interest is enormous.
The Rule of 72
A quick mental shortcut: divide 72 by your interest rate to estimate how many years it takes to double your money.
- At 6% interest: 72 ÷ 6 = 12 years to double
- At 9% interest: 72 ÷ 9 = 8 years to double
- At 12% interest: 72 ÷ 12 = 6 years to double
Compound Interest Working Against You
Compounding isn't always your friend. On debt — especially credit cards — compound interest works the same way in reverse. If you carry a balance at a high interest rate, you're paying interest on interest, and the debt can grow faster than you can pay it down.
This is why paying off high-interest debt is typically more impactful than investing new money: the guaranteed "return" of eliminating expensive debt is hard to beat.
The Most Important Variable: Time
The most powerful factor in compounding isn't the interest rate — it's time. Starting early, even with small amounts, can dramatically outpace starting later with larger contributions. Every year you delay is a year of compounding you can't get back.
This is why financial advisors consistently emphasize starting to save and invest as early as possible, even if the amounts feel insignificant.
Key Takeaways
- Compound interest = earning interest on your interest
- It accelerates growth over time — slowly at first, then dramatically
- More frequent compounding = slightly faster growth
- On debt, compounding works against you
- Time is the most powerful variable — start as early as you can