How interest rates work: A beginner’s guide for borrowers and savers

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What you pay when you borrow; what you receive when you are the lender.

© anuchit2012/stock.adobe.com; Photo illustration Encyclopædia Britannica, Inc.

To understand how interest rates work, you first need to look at the two ways in which they affect you. There’s the rate you pay when you borrow money from a lender, and the interest rate you receive when you deposit money at a bank or credit union.

Interest rates set by lenders cover a variety of loans, such as credit card interest, student loan interest, and mortgage interest. You earn interest when you open a savings account or a certificate of deposit, or when you buy bonds.

Interest rates are calculated in two ways. Simple interest is tallied as a percentage of the principal over time, but compound interest (also called compounding interest) includes accrued interest along with the principal. Most loans and savings deposits use compound interest.

Interest rate calculators can help you understand a loan’s total cost using a compound interest formula. Five figures determine compound interest:

Interest rate calculators can give borrowers a true cost estimate of a loan over time, since they calculate the total amount paid—both principal and interest—for the life of the loan.

Another key term to know is the annual percentage rate (APR), which is how banks and credit card companies advertise loans. APR is the total cost of the loan and can include interest rates and other fees.

Banks may charge a fixed or variable rate. A fixed rate will stay the same over the life of the loan. Conventional mortgages, auto loans, and many student loans are fixed.

Variable rate loans are tied to a benchmark, such as a bank’s prime lending rate—the lowest rate banks lend to their most creditworthy customers. Any changes to that prime rate will change the loan’s interest rate. Banks typically change their prime rate when the Federal Reserve adjusts the fed funds rate. Loans tied to variable rates include adjustable-rate mortgages (ARMs) and credit card debt.

With a fixed rate:

With a variable rate:

You earn interest on money deposited in a savings account, money market account, or certificate of deposit. This interest is described as the annual percentage yield (APY).

Another way to earn interest is to “become a lender” yourself. Municipalities, the federal government, and corporations issue bonds and other fixed-income securities to raise money. When you buy a bond, you’re lending money to the issuer, and they pay you a fixed rate (monthly, quarterly, or annually) over a set time period. At the end of the loan period (at “maturity”), you get back your original investment (the principal).

Many retirees use the fixed payments they receive from bonds as a steady paycheck in retirement.

Compounding gets to the core of borrowing and saving. Compound interest is sometimes referred to as “interest on interest” because it accumulates every pay period and grows exponentially over longer time periods. Compound interest benefits savers, but it makes the true cost of a loan more expensive for borrowers.

Here’s an example of how compound interest works from a savings perspective:

Compound interest can electrify your savings. But if you’re borrowing money, compounding can really be a headwind. Loan rates, particularly those on credit cards and other loans not backed by collateral, are usually substantially higher than savings rates.

Interest rates have a lot of moving parts, and the terminology can be confusing. If you’re a borrower and interest rates are high, your monthly payments will also be high. In other words, if you’re borrowing money to buy something, higher rates make the item—a house, a car, a vacation—more expensive.

It’s worth shopping around to compare interest rates—whether you’re seeking a loan or a place to park your savings.

And remember to monitor your interest as you would tend a garden. A loan is a weed, so cut it down as soon as possible. Your savings are flowers, so let them bloom.