Compound interest explained simply
Compound interest is the mechanism by which your savings earn interest, then that interest itself earns interest, and so on. Over a few months, the effect is invisible. Over twenty or thirty years, it can account for a large share of your final savings. Understanding it is the best argument for starting to save early, even with small amounts.
Simple interest versus compound interest
Imagine $1,000 invested at 5% per year.
- With simple interest, you earn $50 every year: after 10 years, you have $1,500.
- With compound interest, the interest is added to the capital each year: $1,050 after one year, $1,102.50 after two, and about $1,629 after 10 years.
The difference — $129 — comes only from interest earned on previous interest. And it grows faster the longer you wait.
Regular savings: an example
Now let us save $200 a month, with interest calculated monthly:
| Duration | Amount saved | At 3% / year | At 5% / year |
|---|---|---|---|
| 10 years | $24,000 | $27,948 | $31,056 |
| 20 years | $48,000 | $65,660 | $82,207 |
At 5%, after 20 years, interest represents $34,207, or more than 40% of the final total. Doubling the duration from 10 to 20 years more than doubles the result, because the interest has more time to work.
The rule of 72
To estimate how long it takes for money to double, divide 72 by the annual rate. At 3%, money doubles in about 24 years; at 6%, in about 12 years; at 9%, in about 8 years. It is an approximation, but a very useful one for comparing options quickly.
Put this into practice with your own figures.
Open the plannerTime matters more than the rate
Consider two savers. The first puts aside $200 a month from age 25 to 35, then stops. The second starts at 35 and saves $200 a month until 65. With the same rate, the first often ends up with a comparable amount despite saving three times less, because her money has had more years to grow. The exact result depends on the rate, but the lesson is constant: the earlier you start, the less effort you need.
Three precautions
- Inflation: a 3% return with 3% inflation does not increase your purchasing power. What counts is the real return, after inflation.
- Fees: annual fees of 1 to 2% can absorb a large part of the compound effect over the long term. Compare them carefully.
- Risk: higher returns come with fluctuations. Money you need within a few years should not depend on the stock market.
Debts work the same way — against you
Compound interest also applies to debts. A credit card balance at 20% per year grows quickly if only the minimum is paid. This is why paying off expensive debts is often the best “investment” available: it stops compound interest from working against you.
Frequently asked questions on this topic
Is monthly or annual compounding better?
For the same annual rate, more frequent compounding gives a slightly higher result, because interest starts earning interest sooner. The difference is small compared with the effect of the rate, the duration and the regularity of your savings.
Do I need a large amount to benefit from compound interest?
No. Regularity and time matter more than the starting amount. Small monthly amounts saved over many years can grow into a significant sum, as the example in this article shows.
Where can I check the calculations?
You can use our planner (step 5) or the compound interest calculator published by the U.S. Securities and Exchange Commission on Investor.gov. Both use the same principle: each period's return is added to the balance.
What is the difference between nominal and real return?
The nominal return is the rate announced by the bank or the fund. The real return is what remains after inflation: roughly, nominal return minus inflation. If your savings earn 4% while prices rise by 3%, your purchasing power grows by only about 1% a year. Long-term plans should be judged on real returns.
Good to know — Returns are never guaranteed for investments that can fluctuate. Use prudent assumptions — and remember that past performance does not predict future returns.
Try it with your figures
In step 5 of the planner, enter your current savings, an expected annual return and a duration. The chart shows two curves: the amounts you put in, and your savings with interest. The gap between the two is the work of compound interest. The returns shown are assumptions, not promises: use prudent rates.
