Interest is the price you pay to borrow money or the return earned on an investment. For borrowers, interest is most often reflected as an annual percentage of the amount of a loan. This percentage is known as the interest rate on the loan. For investors or savers, interest comes in the form of an annual percentage yield (APY).

For example, a bank will pay you interest when you deposit your money in a high-yield savings account. The bank pays you to hold and use your money to invest in other transactions. Conversely, if you borrow money to pay for a large expense, the lender will charge you interest on top of the amount you borrowed.

How interest works when borrowing

Whenever you borrow money, you are required to pay that base amount (the principal) back to your lender. In addition, you will be required to pay your lender the interest, which is typically an annual percentage of the principal, set for the loan. These loans come in many forms. You may encounter them in the form of credit cards, car loans, mortgages, personal loans and more. Understanding how the interest terms and repayment requirements work is important.

For example, let’s say you borrow $10,000 from your bank in a straightforward loan with a 10 percent interest rate per annum (meaning per year), and the loan is payable in five years. Interest on a typical bank loan is added to monthly payments and is usually compounded monthly. In this example, you’d pay about $2,748.23 in interest over the life of the loan.

You can use Bankrate’s loan calculator to estimate how much interest you would pay on a loan.

How lenders determine interest rates

Typically, banks use a number of different factors to determine your interest rate, including your credit score and debt-to-income ratio, which signal the risk of lending to you. It also depends on the type of lending, such as a credit card or a home loan. On top of this, commercial lenders usually also charge a separate fee for establishing a loan with a customer.

Let’s say you want to apply for a $5,000 loan from your bank. To establish the interest rate it will charge you, your bank must consider what it pays in interest to get the funds it will lend to you (say, 4 percent). The bank will also have loan servicing costs and overhead it will allocate to your interest rate (say, 2 percent). And of course the bank wants to account for default risk and make some profit (say, another 2 percent). To account for these costs, your loan may carry an interest rate around 8 percent.

The difference between interest and compound interest

There are two basic methods to calculate interest: Simple interest and compound interest.

Simple interest
With simple interest, your interest rate payments added into your monthly payments, but the interest doesn’t compound. For example, a five-year loan of $1,000 with simple interest of 5 percent per year would require $1,250 over the life of the loan ($1,000 principal and $250 in interest). You’d calculate the interest by multiplying the principal, the annual percentage rate (APR) and the length of the loan: $1,000 x 0.05 x 5.
Compound interest
This is determined by continually calculating the interest on the principal plus the interest charged for the previous payment period. Compound interest is designed to generate higher returns, at times much higher than simple interest, by compounding the interest earned in the previous terms. If you take out the same loan above but it charges compound interest, you’d pay slightly over $1,332 over the life of the loan ($1,000 principal and $132 in interest).

For large loans with high interest extended over a long term, the increase in total amount paid when interest is compounded can be significant. For this reason, it’s always important to ask your lender or your bank whether a loan or your savings account will have simple or compound interest.

Interest vs. APY

If you’re an investor or saver, understanding APYs — the compounded interest that a financial institution pays you on savings and investments — can help you grow your wealth over time. When you open a savings vehicle, like a savings account or certificate of deposit, the listed APY tells you how much you will earn over a year.

For example, suppose you have a savings account with an APY of 5 percent. That APY accounts for the simple interest rate and the additional interest due to monthly compounding earned in a year. If you had $10,000 in the account, you’d earn $500 in interest after one year.

Bottom line

Interest is a fundamental concept to personal finance. It has a considerable impact on our personal finance decisions, including saving, investing and borrowing. Understanding how interest works, as well as the distinction between simple and compound interest, can help you make informed decisions about how you borrow and save.

— Bankrate’s René Bennett contributed to an update of this story.