What Is Interest?

Custom illustration shows how much to pay (or earn) in interest with a set of hands holding a large coin with a dollar sign on it that is sprouting a plant. Facts include the interest rate, the amount of the loan, and how long it takes to repay

The Balance / Theresa Chiechi


Interest is the cost of using somebody else’s money. The borrower pays interest, and the lender receives it.

Definition and Example of Interest

Interest is calculated as a percentage of a loan (or deposit) balance, paid to the lender periodically for the privilege of using their money. The amount is usually quoted as an annual rate, but interest can be calculated for periods that are longer or shorter than one year.

Interest is additional money that must be repaid in addition to the original loan balance or deposit. To put it another way, consider the question: What does it take to borrow money? The answer: More money.

As an example, if you take out a loan to buy a car, you'll owe the amount of the loan (also called the "principal"), plus the interest, which is the cost the lender charges you for borrowing. If your car loan is for $10,000 at 6% interest, you'll have to repay the $10,000, as well as pay the lender 6% of $10,000 (which is $600), for a total of $10,600 altogether. Your lender might give you many months to repay this loan.

On the other hand, if you deposit money in a savings account, you can be the one who earns interest. If you deposit $10,000 in an account that earns 6% interest, you'll not only keep your $10,000, but you'll earn an additional $600 in interest, too, so you'll end up with $10,600 altogether.

How Does Interest Work?

There are several different ways to calculate interest, and some methods are more beneficial for lenders. The decision to pay interest depends on what you get in return, and the decision to earn interest depends on the alternative options available for investing your money.

When Borrowing

To borrow money, you’ll need to repay what you borrow. In addition, to compensate the lender for the risk of lending to you (and their inability to use the money anywhere else while you use it), you need to repay more than you borrowed.

When Lending

If you have extra money available, you can lend it out yourself or deposit the funds in a savings account, effectively letting the bank lend it out or invest the funds. In exchange, you’ll expect to earn interest. If you are not going to earn anything, you might be tempted to spend the money instead, because there’s little benefit to waiting.

How much do you pay or earn in interest? It depends on:

  1. The interest rate
  2. The amount of the loan
  3. How long it takes to repay

A higher rate or a longer-term loan results in the borrower paying more.

For example, an interest rate of 5% per year and a balance of $100 result in interest charges of $5 per year, assuming you use simple interest. To see the calculation, use the Google Sheets spreadsheet with this example. Change the three factors listed above to see how the interest cost changes.

Most banks and credit card issuers do not use simple interest. Instead, interest compounds, resulting in interest amounts that grow more quickly.

Do I Have to Pay Interest?

When you borrow money, you generally have to pay interest. That might not be obvious, though, as there’s not always a line-item transaction or separate bill for interest costs.

Installment debt

With loans like standard home, auto, and student loans, the interest costs are baked into your monthly payment. Each month, a portion of your payment goes toward reducing your debt, but another portion is your interest cost. With those loans, you pay down your debt over a specific time period (a 15-year mortgage or five-year auto loan, for example).

Revolving debt

Other loans are revolving loans, meaning you can borrow more month after month and make periodic payments on the debt. For example, credit cards allow you to spend repeatedly as long as you stay below your credit limit.

Interest calculations vary. Refer to your loan agreement to figure out how interest is charged and how your payments work.

Additional costs

Loans are often quoted with an annual percentage rate (APR). This number tells you how much you pay per year and may include additional costs above and beyond the interest charges. Your pure interest cost is the interest rate (not the APR). With some loans, you pay closing costs or finance costs, which are technically not interest costs that come from the amount of your loan and your interest rate. It would be useful to find out the difference between an interest rate and an APR. For comparison purposes, an APR is usually a better tool.

How Do I Earn Interest?

You earn interest when you lend money or deposit funds into an interest-bearing bank account such as a savings account or a certificate of deposit (CD). Banks do the lending for you: They use your money to offer loans to other customers and make other investments, and they pass a portion of that revenue to you in the form of interest.

Periodically (every month or quarter, for example), the bank pays interest on your savings. You’ll see a transaction for the interest payment, and you’ll notice that your account balance increases. You can either spend that money or keep it in the account so it continues to earn interest. Your savings can really build momentum when you leave the interest in your account. You’ll earn interest on your original deposit as well as on the interest added to your account.

Earning interest on top of the interest you earned previously is known as "compound interest."

For example, suppose you deposit $1,000 in a savings account that pays a 5% interest rate. With simple interest, you’d earn $50 over one year. To calculate:

  1. Multiply $1,000 in savings by 5% interest.
  2. $1,000 x .05 = $50 in earnings (see how to convert percentages and decimals).
  3. Account balance after one year = $1,050.

However, most banks calculate your interest earnings every day, not just after one year. That works out in your favor, because you take advantage of compounding. Assuming your bank compounds interest daily:

  • Your account balance would be $1,051.16 after one year.
  • Your annual percentage yield (APY) would be 5.12%.
  • You would earn $51.16 in interest over the year.

The difference might seem small, but we’re only talking about your first $1,000. With every $1,000, you’ll earn a bit more. As time passes, and as you deposit more, the process will continue to snowball into bigger and bigger earnings. If you leave the account alone, you’ll earn $53.78 in the following year, compared to $51.16 the first year.

See a Google Sheets spreadsheet with an example of compound interest. Make a copy of the spreadsheet, and make changes to learn more about compound interest.

Key Takeaways

  • Interest is the money you either owe when borrowing or are paid when lending money.
  • The amount is calculated as a percentage of the loan.
  • To earn interest, lend money or deposit funds into an interest-bearing bank account.
  • Earning interest on top of the interest you earned previously is known as "compound interest."

Article Sources

  1. Council of Development Finance Agencies. "Revolving Loan Funds & Development Finance." Accessed Dec. 6, 2021.