How your bank calculates the interest you earn

Your bank pays you interest based on three things: how much money you have in the account, what annual percentage yield (APY) the bank is offering, and how long your money sits there. The bank multiplies your balance by the APY, then divides by the number of days in a year to figure out what you earn each day. Most banks add that daily interest to your account monthly, though some do it quarterly or annually.

The simplest way to see what you will earn is to use your bank's interest calculator on their website, or to ask a teller. But understanding the math behind it helps you compare accounts and spot when a bank's offer is actually good.

Key Takeaways

  • Interest is calculated daily based on your balance and the APY, but credited to your account monthly, quarterly, or annually depending on the bank.
  • APY includes the effect of compounding — when interest earns interest — so it is always higher than the straightforward interest rate.
  • A higher APY matters most when you have a larger balance or plan to leave money untouched for months or years.
  • Your bank must disclose the APY in writing before you open the account, so you can compare offers from different banks.

The difference between APY and interest rate

Banks advertise APY, not just an interest rate, because APY tells you the real amount you will earn. The interest rate is the percentage the bank pays on your balance. APY is that rate plus the effect of compounding — when the interest you earn starts earning interest too.

Here is a concrete example. Suppose a bank offers 4% APY on a savings account. You deposit $1,000 and leave it untouched for one year. At the end of the year, you have $1,040. That $40 is your earnings. If the bank had used straightforward interest instead of compounding, you would earn the same $40 in this case — but with compounding, if you leave the money for two years, the second year's interest is calculated on $1,040, not $1,000, so you earn slightly more.

The difference is small with savings accounts because most banks compound daily or monthly. But it matters more the longer you leave the money and the higher the APY. Always compare APY, not the interest rate, when you are deciding between banks.

How daily interest compounds into monthly deposits

Banks calculate interest every single day. They take your account balance at the end of each day, multiply it by the APY, and divide by 365 (or 366 in a leap year). That gives them your interest for that one day. They do this for every day of the month, then add all those daily amounts together and deposit the total into your account.

This matters because your balance changes throughout the month. If you deposit $500 on the 15th, you only earn interest on that $500 from the 15th onward. If you withdraw $200 on the 20th, your balance drops and you earn less interest for the rest of the month. The bank tracks all of this automatically.

Some banks use a different method called the "average daily balance." They add up your balance at the end of each day, divide by the number of days in the month, and use that average to calculate interest. The result is usually very similar to daily compounding, but it can matter slightly if your balance swings a lot.

What APY rates look like across different banks

APY varies widely depending on the type of account and the bank. High-yield savings accounts at online banks often offer between 4% and 5% APY, while traditional brick-and-mortar banks may offer 0.01% to 0.5%. Money market accounts and certificates of deposit (CDs) sometimes offer higher rates than savings accounts at the same bank.

APY also changes over time. Banks raise or lower their rates based on what the Federal Reserve does with interest rates. When the Fed raises rates, banks usually raise APY on savings accounts within weeks. When the Fed lowers rates, banks lower APY too — sometimes when ready.

Your bank must show you the APY in writing before you open the account. Look for a document called the "Truth in Savings Act Disclosure" or "Account Disclosure." It lists the APY, how often interest is compounded, and when it is credited to your account. Keep this document so you can verify the rate later.

Calculating what you will earn over time

To estimate your earnings, use this straightforward formula: multiply your balance by the APY, then divide by 12 (for a monthly estimate). For example, if you have $5,000 and the APY is 4%, you earn roughly $5,000 × 0.04 ÷ 12 = $16.67 per month. Over a year, that is about $200.

This formula assumes your balance stays the same and does not account for the exact effect of daily compounding, but it is close enough for planning. For a more precise number, use your bank's online calculator or ask them directly. Many banks have a tool on their website where you enter your balance and it shows you the projected earnings.

If you are comparing two banks, calculate the earnings at each one using the same balance and time period. The difference may seem small month to month, but over a year or longer it adds up. A 4% APY on $10,000 earns about $400 per year, while 0.5% earns about $50 — a difference of $350 annually.

Why your actual earnings might differ from the advertised APY

The APY your bank advertises assumes your balance stays the same for the entire year. In real life, your balance changes when you deposit or withdraw money. If you deposit $1,000 in January and withdraw it in June, you only earn interest on that $1,000 for six months, not twelve.

Also, some banks offer a promotional APY for a limited time — for example, 4.5% for the first three months, then 4% after that. Read the fine print to see when the rate changes. Your bank will send you a notice before the promotional period ends, but it is worth checking your account statement to confirm the new rate.

Finally, if your account balance drops below a minimum, some banks lower the APY or charge a monthly fee that eats into your interest. Check your account agreement to see if there is a minimum balance requirement and what happens if you fall below it.

Frequently Asked Questions

Do I have to pay taxes on savings account interest?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a form called a 1099-INT at the end of the year if you earned $10 or more in interest. You report this on your tax return. The amount is usually small, but it still counts as income.

Is interest better in a savings account or a money market account?

Money market accounts often offer a slightly higher APY than savings accounts at the same bank, but they usually require a higher minimum balance and limit how many withdrawals you can make per month. If you need to access your money frequently, a savings account is simpler. If you have a larger balance and do not need to withdraw often, a money market account may earn you a bit more.

What happens to my interest if I close the account?

You keep all the interest you have already earned. If you close the account on the 15th of the month, you earn interest through the 15th, and that interest is either deposited before the account closes or paid to you separately. Check with your bank about their specific process.

Can the bank change the APY after I open the account?

Yes. Banks can change APY at any time, and they usually notify you by mail or email before the change takes effect. You are not locked into the rate you saw when you opened the account. This is why it is worth shopping around every few months if rates are changing.

How is interest different on a CD?

A CD (certificate of deposit) locks your money away for a set period — three months, one year, five years, and so on. In exchange, the bank usually offers a higher APY than a savings account. But if you withdraw the money before the term ends, you pay a penalty. A savings account lets you withdraw anytime without penalty, so the APY is lower.