What Is Compound Interest?
Compound interest is interest earned on interest. Unlike simple interest (which only earns on the original amount), compound interest grows exponentially because each period's interest is added to the principal.
Albert Einstein reportedly called it "the eighth wonder of the world." Whether he actually said that or not, the math backs it up.
The Compound Interest Formula
A = P(1 + r/n)^(nt)
Where:
- A = Final amount
- P = Principal (starting amount)
- r = Annual interest rate (decimal)
- n = Number of times compounded per year
- t = Number of years
Real-World Example
Let's say you invest $10,000 at 7% annual return (the stock market's historical average after inflation):
| Years | Balance | Interest Earned |
|---|---|---|
| 5 | $14,026 | $4,026 |
| 10 | $19,672 | $9,672 |
| 15 | $27,590 | $17,590 |
| 20 | $38,697 | $28,697 |
| 25 | $54,274 | $44,274 |
| 30 | $76,123 | $66,123 |
Your $10,000 grew to over $76,000 — and you earned $66,123 in interest alone. That's the power of compounding.
Why Starting Early Matters So Much
Consider two people:
- Alex starts investing $200/month at age 25 and stops at 35 (10 years, $24,000 invested)
- Jordan starts investing $200/month at age 35 and continues until 65 (30 years, $72,000 invested)
At 7% annual returns:
- Alex at age 65: ~$353,000
- Jordan at age 65: ~$227,000
Alex invested one-third the money but ended up with more — because those extra 10 years of compounding made all the difference.
How Compounding Frequency Affects Returns
$10,000 at 7% for 10 years:
| Frequency | Final Balance |
|---|---|
| Annually | $19,672 |
| Quarterly | $19,898 |
| Monthly | $19,967 |
| Daily | $20,137 |
More frequent compounding = slightly more growth. Most savings accounts compound daily; investments compound based on returns.
How to Maximize Compound Interest
- Start now — Time is the most important variable. Even $50/month matters.
- Be consistent — Regular contributions accelerate growth dramatically.
- Reinvest dividends — Don't take earnings out; let them compound.
- Minimize fees — A 1% annual fee might sound small, but it can cost hundreds of thousands over a career.
- Increase contributions — Bump up your savings rate with every raise.
Try It Yourself
Use our [compound interest calculator](/compound-interest-calculator) to see exactly how your money can grow based on your starting amount, monthly contributions, and expected returns.