What Is Compound Interest?
If you want to understand why long-term investing grows so much faster after the first few years, compound interest is the reason.
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. In simple terms: you earn interest on your interest.
This is different from simple interest, which only applies to the original principal.
The Formula
A = P(1 + r/n)^(nt)
Where:
- A = final amount
- P = principal (initial investment)
- r = annual interest rate (decimal)
- n = number of times interest compounds per year
- t = number of years
Try It Yourself
See how your money could grow with our compound interest calculator:
Simple Interest vs. Compound Interest
Let's compare $10,000 invested at 7% annual interest over 30 years:
| Year | Simple Interest | Compound Interest | Difference |
|---|---|---|---|
| 1 | $10,700 | $10,700 | $0 |
| 5 | $13,500 | $14,026 | $526 |
| 10 | $17,000 | $19,672 | $2,672 |
| 20 | $24,000 | $38,697 | $14,697 |
| 30 | $31,000 | $76,123 | $45,123 |
After 30 years, compound interest produces more than double what simple interest does. That's the power of exponential growth.
The Rule of 72
Want a quick way to estimate how long it takes to double your money? Divide 72 by your interest rate:
Years to double = 72 ÷ interest rate
Examples:
- At 6% → 72 ÷ 6 = 12 years
- At 8% → 72 ÷ 8 = 9 years
- At 10% → 72 ÷ 10 = 7.2 years
- At 12% → 72 ÷ 12 = 6 years
How Compounding Frequency Matters
The more frequently interest compounds, the more you earn. Here's $10,000 at 8% annual rate over 10 years:
| Compounding | Final Amount | Interest Earned |
|---|---|---|
| Annually | $21,589 | $11,589 |
| Quarterly | $21,911 | $11,911 |
| Monthly | $22,196 | $12,196 |
| Daily | $22,253 | $12,253 |
The difference between annual and daily compounding is $664 — not huge, but it adds up over longer time periods and larger amounts.
Why Starting Early Matters More Than Starting Big
Consider two investors:
Alice starts investing $200/month at age 25, stops at 35 (10 years, $24,000 total invested).
Bob starts investing $200/month at age 35, continues until 65 (30 years, $72,000 total invested).
Assuming 8% annual returns:
- Alice at 65: $314,870 (from $24,000 invested)
- Bob at 65: $298,072 (from $72,000 invested)
Alice invested one-third as much money but ended up with more — because compound interest had 10 extra years to work.
Where to Find Compound Interest
Compound interest works in many financial products:
It Works FOR You
- Savings accounts — typically compound daily or monthly
- CDs (Certificates of Deposit) — fixed rate with guaranteed compounding
- Index funds and ETFs — reinvested dividends compound over time
- Bonds — reinvested coupon payments
It Works AGAINST You
- Credit card debt — compounds daily at 15-25% APR
- Student loans — interest capitalizes if unpaid
- Mortgages — most of early payments go to interest
The key insight: make compound interest your ally by investing early and avoiding high-interest debt.
Key Takeaways
- Compound interest means earning interest on interest — it grows exponentially
- The Rule of 72 gives a quick estimate for doubling time
- More frequent compounding = slightly more growth
- Starting early matters more than investing more money later
- Compound interest works both for (investments) and against you (debt)
- Even small regular contributions can grow into significant amounts over decades