Compound interest is interest earned on your interest. You put money in, it earns a return, and then that return earns its own return, and so on. It sounds small. Given enough time, it is the difference between money that trickles and money that snowballs.
What compound interest actually means
The US regulator investor.gov defines it in seven words: interest paid on principal and on accumulated interest. Your principal is the money you start with. In year one you earn interest on that. In year two you earn interest on the principal plus the interest from year one. The base you are earning on keeps quietly getting bigger, which means each year adds a little more than the last.
Contrast that with simple interest, where you only ever earn on the original amount. The gap between the two starts invisibly small and ends up enormous, because compounding feeds on itself while simple interest just repeats.
Simple vs compound: a worked example
Put 1,000 away at 8% a year for 30 years. With simple interest you would earn 80 every year, 2,400 in total, ending with 3,400. With compound interest, the same 1,000 at the same rate grows to about 10,063. Same money in, same rate, same time. The only difference is that compounding let the interest earn interest.
The three things that drive it
Compound growth has exactly three inputs, and they are not equally important.
- How much you put in. Obvious, and the one people focus on. It helps, but it is the weakest lever on its own.
- The rate of return. A higher rate compounds faster. But chasing rate usually means taking on more risk, so this lever has a cost.
- Time. The quiet giant. Because the curve bends upward, an extra decade at the start is worth more than a much larger sum added at the end. This is why starting early, even with small amounts, beats starting late with large ones.
The Rule of 72: a shortcut for your head
You do not need a calculator to estimate compounding. The Rule of 72, endorsed by regulators as a rough guide, says: divide 72 by the annual interest rate, and you get roughly the number of years for your money to double. It is an approximation, most accurate for rates between about 6% and 10%, but it is close enough to be useful in a conversation.
| Annual rate | 72 ÷ rate | Years to double |
|---|---|---|
| 2% | 72 ÷ 2 | ≈ 36 years |
| 4% | 72 ÷ 4 | ≈ 18 years |
| 6% | 72 ÷ 6 | ≈ 12 years |
| 8% | 72 ÷ 8 | ≈ 9 years |
| 10% | 72 ÷ 10 | ≈ 7.2 years |
The table makes the cost of a low rate visible. Money in a typical everyday savings account, often earning well under 1%, would take the better part of a century to double. The same money at long-run stock-market rates could double in well under a decade. The rate is not a detail.
It cuts both ways
Compounding is not on your side by default. It follows the money. Carry a balance on a credit card at 20-something percent and that interest compounds against you, on the interest you already owe. This is the same engine that builds wealth, running in reverse. It is why clearing high-interest debt is often the best "investment" available: paying off a 22% card is a guaranteed 22% return, which almost no investment can promise.
Common questions
What is compound interest in simple terms?
It is interest you earn on your interest. Instead of only earning on the money you first put in, you also earn on the returns that money has already generated. Over time, that makes your balance grow faster and faster.
What is the difference between simple and compound interest?
Simple interest is calculated only on your original amount, so you earn the same figure every period. Compound interest is calculated on your original amount plus all the interest earned so far, so each period earns a little more than the last. Over long periods the compound total is dramatically higher.
How does the Rule of 72 work?
Divide 72 by your annual interest rate to estimate how many years it takes your money to double. At 6% it is about 12 years; at 8%, about 9 years. It is an approximation that is most accurate for rates between roughly 6% and 10%.
Does compound interest work against you with debt?
Yes. On borrowing such as credit cards, interest compounds on what you owe, including interest already added. That is why high-interest debt grows quickly and why clearing it is often more valuable than investing.
How often does interest compound?
It depends on the account. Interest can compound annually, monthly, or daily. More frequent compounding grows money slightly faster at the same headline rate, though the effect is small compared with the impact of the rate itself and the length of time invested.
This article is educational and not financial advice. Investment returns are not guaranteed and the value of investments can fall as well as rise. For decisions tied to your circumstances, speak to a regulated professional in your jurisdiction.
- 1.Compound Interest (definition) — US Securities and Exchange Commission — Investor.gov
- 2.Compound Interest Calculator — US Securities and Exchange Commission — Investor.gov
- 3.Doubling Your Money: The Rule of 72 — Nebraska Department of Banking & Finance
- 4.Historical Returns on Stocks, Bonds and Bills (1928–2025) — NYU Stern — Aswath Damodaran, 2026
- 5.The Ultimate Quotable Einstein / Quote Investigator on the "eighth wonder" line — Quote Investigator, 2019