Banks pay you interest on savings, and you pay interest on borrowed money

Bank interest is the cost of borrowing money, or the reward for lending it. When you put money in a savings account, the bank borrows it from you and pays you interest—a percentage of what you deposited. When you borrow from a bank through a loan or credit card, you pay interest back to them. The percentage rate stays the same, but the dollar amount changes based on how much money is involved and how long the money sits in the account or loan.

The bank makes money on the difference between what it pays you and what it charges borrowers. If a savings account pays 4% annual interest and the bank lends that same money out at 7%, the bank keeps the 3% gap. This is how banks stay in business—they are middlemen between savers and borrowers.

Key Takeaways

  • Interest is calculated as a percentage of your balance, and the rate is usually stated as an annual percentage rate (APR) even if interest compounds monthly or daily.
  • Simple interest multiplies the rate by the principal once; compound interest multiplies it by the growing balance, so you earn interest on your interest.
  • The longer money stays in an account or loan, the more interest accumulates, which is why time is as important as the rate itself.
  • Banks advertise rates that vary by account type, deposit size, and current market conditions, so comparing rates across banks can add hundreds of dollars to your savings over years.

How interest rates are expressed and what they mean

Banks state interest as an annual percentage rate (APR), which is the percentage you earn or owe in one year. A savings account offering 4.5% APR means that if you keep $1,000 in the account for a full year with no deposits or withdrawals, you will earn $45 in interest. A credit card charging 18% APR means that if you carry a $1,000 balance for a year, you will owe $180 in interest charges.

The APR is standardized so you can compare rates across different banks and account types. However, the actual interest you earn or pay depends on three things: the rate, the balance, and the time the money sits there. A higher rate is better when you are saving, and lower is better when you are borrowing. But even a low rate on a large balance or over a long time can add up to real money.

Simple interest versus compound interest

Simple interest is calculated once on the original amount you deposited or borrowed. If you put $1,000 in a simple-interest savings account at 5% APR, you earn $50 in year one. In year two, you still earn $50 on the original $1,000—the interest does not grow. Simple interest is rare in consumer banking today.

Compound interest is calculated on your balance plus any interest already earned. After year one at 5% APR, your $1,000 becomes $1,050. In year two, the bank calculates 5% on $1,050, not $1,000, so you earn $52.50. The interest compounds—you earn interest on your interest. Over decades, this difference becomes enormous. A $10,000 deposit at 5% simple interest grows to $15,000 in ten years. The same deposit at 5% compound interest grows to $16,289.

Banks compound interest at different intervals: daily, monthly, or quarterly. Daily compounding is best for savers because interest is calculated and added to your balance every day, so you earn interest on a slightly larger balance each time. The difference between daily and monthly compounding is small on a savings account but adds up over years.

How banks calculate interest on savings accounts

When you open a savings account, the bank tells you the APR and how often it compounds. To find out how much you will actually earn, you need to know the balance, the rate, and the compounding frequency. Most online banks compound daily and publish the effective annual rate (APY), which shows what you will earn after compounding is factored in. A 4.5% APR compounded daily becomes about 4.6% APY.

Your balance changes every time you deposit or withdraw money, so the bank recalculates interest on the new balance. If you deposit $5,000 on the first of the month and withdraw $2,000 on the fifteenth, the bank calculates interest on $5,000 for fifteen days and $3,000 for the remaining days. Some banks use the average daily balance method, which adds up your balance each day and divides by the number of days in the month—this smooths out the effect of deposits and withdrawals.

Interest is usually credited to your account monthly, though some banks credit it daily. When interest is credited, it becomes part of your balance and begins earning interest itself. This is why leaving money untouched in a savings account compounds your wealth over time.

How interest works on loans and credit cards

When you borrow money, you pay interest to the lender. The calculation is the same as savings—a percentage of the balance—but the direction is reversed. You owe the interest instead of earning it. A $10,000 car loan at 6% APR costs you $600 in interest over the first year, though the actual amount varies depending on how much of the loan you have paid back.

Credit cards charge interest on the outstanding balance if you do not pay the full amount by the due date. If your card has an 18% APR and you carry a $2,000 balance, you will owe about $30 in interest that month (18% divided by 12 months). If you only make the minimum payment and the balance stays near $2,000, you will pay $30 or more every month until the balance is gone. This is why credit card debt grows so quickly—interest compounds on the unpaid balance, and if you only pay the minimum, most of your payment goes to interest, not principal.

Loans like mortgages and car loans use amortization, which means you pay a fixed amount each month that covers both principal and interest. Early payments are mostly interest; later payments are mostly principal. A 30-year mortgage at 6% APR means you will pay roughly the same amount in interest as the original loan amount—a $300,000 mortgage costs about $300,000 in interest over thirty years.

Why interest rates change and how to find the best rate

Banks set their interest rates based on the federal funds rate, which the Federal Reserve adjusts to manage inflation and economic growth. When the Fed raises rates, banks raise the rates they pay on savings and charge on loans. When the Fed lowers rates, banks do the same. This is why savings account rates jumped from near zero in 2021 to 4% or higher in 2023—the Fed raised rates to fight inflation.

Different banks offer different rates on the same type of account because they have different costs and different strategies. Online banks typically offer higher savings rates than brick-and-mortar banks because they have lower overhead. Credit unions often offer better rates on loans and savings than traditional banks. Comparing rates across at least three banks before opening an account or taking out a loan can mean hundreds of dollars in difference over the life of the account or loan.

Some banks offer promotional rates—a higher rate for a limited time to attract new customers. These rates are real, but they usually drop after three to six months. Read the fine print to see when the promotional rate ends and what the standard rate will be.

The relationship between principal, rate, and time

Interest depends on three variables: the principal (the amount of money), the rate (the percentage), and the time (how long the money sits there). Change any one of them and the interest changes. A $5,000 deposit at 4% for one year earns $200. The same $5,000 at 5% for one year earns $250. The same $5,000 at 4% for two years earns roughly $408 because of compounding.

This is why time is so powerful in savings. A 25-year-old who saves $200 a month at 5% interest will have roughly $200,000 by age 65, even if they never increase the monthly amount. A 45-year-old who saves the same $200 a month at the same rate will have only about $60,000 by age 65. The extra twenty years of compounding makes the difference. On the borrowing side, this is why paying off debt faster saves money—less time means less interest accumulates.

Frequently Asked Questions

How often should I check my interest rate to see if I should move my money?

Check rates when you are opening a new account or when the Fed has recently changed rates. Rates move slowly and in the same direction across most banks, so switching accounts every month is not worth the effort. If your current account is paying 2% and competitors are paying 4%, moving makes sense. If the difference is 0.1%, it does not.

Does interest compound on money I withdraw before the year is over?

Yes. Interest compounds on whatever balance you have at each compounding period. If you deposit $1,000 and withdraw $500 after six months, you earn interest on $1,000 for six months and $500 for the remaining six months. The interest you already earned stays in the account and compounds with the remaining balance.

Why do credit cards charge more interest than banks charge on loans?

Credit cards are unsecured debt—the bank has no collateral if you do not pay. A car loan is secured by the car, so the bank can repossess it if you default. The higher risk of credit card lending means higher interest rates. Credit card companies also make money from merchant fees, which allows them to offer rewards, but the interest rate itself reflects the risk.

Can I negotiate my interest rate with a bank?

On savings accounts, no—rates are set by the bank and apply to all customers with that account type. On loans and credit cards, sometimes yes. If you have good credit and a long history with the bank, you can ask for a lower rate, especially on mortgages and car loans. The worst they can say is no, and asking costs nothing.

What is the difference between APR and APY?

APR is the annual percentage rate before compounding is factored in. APY is the annual percentage yield after compounding. If a savings account offers 4.5% APR compounded daily, the APY will be slightly higher—around 4.6%—because you earn interest on your interest. Banks must disclose both numbers so you can compare accounts fairly.