Compound Interest Explained

Compound interest is interest calculated not only on the original amount of money, called the principal, but also on the interest that has already been added. In simple terms, your money can earn a return, and then that return can itself start earning a return.

This is different from simple interest, which is calculated only on the original principal each period. Over short periods the difference between simple and compound interest can be small, but over longer periods it can become significant.

Suppose you deposit $1,000 into an account that earns 5% interest per year, compounded annually, and you make no additional deposits or withdrawals. After year one, you would have $1,050 ($1,000 plus 5% of $1,000). After year two, the 5% is calculated on $1,050, not just the original $1,000, giving you $1,102.50.

By year three, you would have about $1,157.63. Notice that the dollar amount of interest earned increases slightly each year, even though the interest rate stayed the same, because the base amount it is calculated on keeps growing.

Because each year's growth builds on the previous year's total, the effect of compounding becomes more noticeable the longer money is left to grow. A dollar invested at a young age has more time to compound than a dollar invested later, even if the rate of return is identical.

For example, using a hypothetical 6% annual return, $1,000 grows to roughly $1,791 after 10 years, but to roughly $3,207 after 20 years, and roughly $5,743 after 30 years. The growth in the second decade is larger in dollar terms than the growth in the first decade, and the third decade is larger still, even though the rate never changed.

Interest can compound on different schedules: annually, monthly, daily, or other periods. More frequent compounding periods generally produce slightly higher totals for the same stated annual rate, because interest gets added to the balance sooner and starts earning its own interest sooner.

For most everyday savings and loan products, the difference between monthly and daily compounding is usually modest, but it is worth checking how a product compounds when comparing two options with similar stated rates.

The same mathematical principle that grows savings can also grow debt. Credit card balances that are not paid off typically accrue interest on the remaining balance, and if that interest is not paid, it can be added to the balance that future interest is calculated on.

This is one reason carrying a balance on a high-interest credit card can become expensive over time: the interest is compounding against you rather than for you. Understanding this can make the case for paying off high-interest debt promptly, since delay allows the balance to compound further.

A common mistake is underestimating how much regular contributions matter alongside compounding. Even modest, consistent deposits added to a compounding account can meaningfully increase the total over time compared to a single deposit left alone.

Another mistake is assuming a fixed rate of return will apply exactly as shown in an example. Real investment returns vary year to year and are not guaranteed, so examples using a steady hypothetical rate are meant to illustrate the mechanics, not to predict actual results.

Try running a few simple compound interest examples with different time horizons and rates to see how the numbers change, using a basic calculator or spreadsheet. Consider how starting to save or invest earlier, even in small amounts, interacts with the length of time money has to compound.