Compound Interest, Explained
Compound interest means earning returns on your returns: interest calculated not just on your original deposit, but on all the interest it has already earned. It's the reason time matters more than timing in investing.
Reviewed October 2026
Key takeaways
- Compound interest = earning returns on your returns: A = P(1 + r/n)nt.
- Starting early beats investing more: $5,000/yr for 10 years from age 25 grows to about $602,000 by 65 at 7% — more than $5,000/yr for 30 years starting at 35 ($540,000).
- The Rule of 72: divide 72 by the annual return to estimate doubling time (about 9 years at 8%).
- Compounding works against you too: unpaid debt grows exponentially the same way.
The formula
A = final amount · P = principal · r = annual rate (as a decimal) · n = compounding periods per year · t = years
Every period, your balance gets multiplied by (1 + r/n). The base keeps growing, so the dollar gains speed up. That's the whole trick: growth is exponential, not linear. A straight line would be nice and predictable. Real compounding curves upward.
Why starting early beats investing more
Here's the comparison that sells it. Two investors, both earning 7% a year:
| Investor | Invests | Total contributed | Value at 65 |
|---|---|---|---|
| Early, age 25–35 | $5,000/yr for 10 yrs | $50,000 | $602,000 |
| Late, age 35–65 | $5,000/yr for 30 yrs | $150,000 | $540,000 |
Read that again. The early investor put in one-third the money and still finished ahead. Those 10 extra years of compounding outworked 20 extra years of contributions. If you take one idea from this whole guide, make it this one: starting early beats investing more, and it's not close.
Does compounding frequency matter?
Take $10,000 at 7% for 20 years:
| Compounds | Final value |
|---|---|
| Annually | $38,697 |
| Monthly | $40,387 |
| Daily | $40,548 |
Daily beats annual by about 5%. Nice, but honestly? An extra couple of years, or a slightly higher rate, swamps that difference completely. So don't shop for compounding frequency. Shop for time in the market and low fees.
The Rule of 72
Here's the shortcut I actually use: divide 72 by your annual return to estimate doubling time. At 7%, money doubles roughly every 10 years (72 ÷ 7 ≈ 10.3). At 10%, roughly every 7 years. It holds up for rates between about 4% and 12%, which covers most real-world investing.
Compounding works against you, too
Now flip it. Credit card debt at 24% APR compounds exactly the same way, just in reverse. A $5,000 balance on minimum payments can take over a decade to clear, and you'll pay nearly as much in interest as the original debt. The math doesn't care which direction it's pointed. That's not a metaphor. It's the same formula.
See your own growth curve
Plug in your balance, your monthly contributions, and the return you're hoping for. Then watch the curve bend upward, year by year.
Open the Compound Interest CalculatorFrequently asked questions
What is compound interest in simple terms?
Earning interest on your interest. Each period your balance grows, and the next period's interest is calculated on the bigger balance.
What's the compound interest formula?
A = P(1 + r/n)nt — final amount equals principal times one plus the per-period rate, raised to the number of periods.
How often should interest compound?
More frequent is slightly better, but the difference between monthly and daily is small. Time and rate dominate.
What is the Rule of 72?
72 ÷ annual return ≈ years to double your money. At 8%, about 9 years.
Can I lose money with compound interest?
Compounding amplifies whatever the return is — including negative years in the market. Over long periods, diversified market returns have historically been positive, but there are no guarantees.
Educational content, not financial advice. Investment returns vary and past performance doesn't guarantee future results.