The basic formula: multiply your balance by the rate, then divide by the number of days in a year

Banks calculate interest on savings accounts using a straightforward multiplication. Take the money you have in the account, multiply it by the annual interest rate the bank is offering you, then divide by 365 (or sometimes 360, depending on the bank). That gives you the interest earned in one day. The bank repeats this calculation every single day, adding a tiny bit of interest each time.

Here is a concrete example. If you have $1,000 in an account earning 4.5% annual interest, the math looks like this: $1,000 × 0.045 ÷ 365 = $0.12 per day. After 30 days, you would earn about $3.70 in interest (though the actual amount varies slightly because the bank is calculating on a changing balance as interest gets added).

The reason banks do this daily is that compound interest — interest earned on your interest — adds up faster when it happens more often. Money sitting in your account grows not just from your deposits, but from the interest the bank has already paid you.

Key Takeaways

  • Daily interest calculation means the bank multiplies your balance by the annual rate and divides by 365 each day, so your balance grows even when you do not deposit money.
  • The annual percentage yield (APY) already includes the effect of compound interest, so you can compare it directly between banks without doing extra math.
  • Your actual interest earned depends on your lowest balance during the period, because most banks use the "average daily balance" or "lowest balance" method.
  • Interest is usually credited to your account monthly, but the bank calculates it daily, so moving money in or out changes what you earn that month.
  • Online banks typically offer higher interest rates than brick-and-mortar banks, sometimes two to three times higher for the same type of account.

Why the annual percentage yield (APY) matters more than the interest rate

Banks publish two different numbers: the interest rate (also called the annual percentage rate or APR) and the annual percentage yield (APY). The APY is the number you should use to compare accounts, because it already includes the effect of compound interest.

If a bank tells you the interest rate is 4.5%, that is the straightforward rate before compounding. The APY will be slightly higher — maybe 4.60% — because it shows what you actually earn when interest gets added to your balance and then earns interest itself. When you are looking at two savings accounts, comparing their APYs tells you which one will put more money in your pocket without doing any math yourself.

Banks are required by law to show you the APY prominently, so you should see it on the account details page or in the account agreement. If you only see an interest rate and no APY, ask the bank for the APY before opening the account.

How the bank decides which balance to use for calculating interest

Your balance changes every time you deposit or withdraw money, so the bank has to pick a method for deciding which balance to use in the interest calculation. The most common method is average daily balance. The bank adds up your balance at the end of each day during the month, then divides by the number of days. That average is what they use to calculate your interest for the month.

Some banks use the lowest balance method instead, which means they use the smallest amount you had in the account during the month. This method pays you less interest if your balance fluctuates, because even one day with a low balance pulls down the whole month's calculation. A few banks use the highest balance method, which is rare and more favorable to you.

Your account agreement or the bank's website should tell you which method they use. If you are comparing two accounts with the same APY, the one using average daily balance will pay you more if your balance changes during the month.

When interest gets added to your account

Banks calculate interest every day, but they do not add it to your account every day. Most savings accounts have interest credited (added) once a month, usually on the last day of the month or the first day of the next month. Some accounts credit interest quarterly (four times a year) or even annually.

The timing matters because once interest is credited, it becomes part of your balance and starts earning interest itself. If your bank credits monthly, you get twelve compounding events per year. If they credit quarterly, you get four. More frequent crediting means slightly more total interest earned, though the difference is small for typical savings account balances.

You can see when interest was credited by looking at your account statement or transaction history. It usually appears as a deposit labeled "interest paid" or "interest credit" on the day it hits your account.

How to estimate your interest earnings for a year

If you want to know roughly how much interest you will earn in a year without waiting to see it happen, use the APY and multiply. Take your average balance for the year, multiply it by the APY (as a decimal), and that is your estimated annual interest.

For example: if you keep an average of $5,000 in an account with a 4.5% APY, you would earn about $225 in a year ($5,000 × 0.045 = $225). This is an estimate because your actual balance may vary month to month, but it gives you a realistic picture of what the account will earn.

If you want to be more precise, you can calculate month by month using your actual balance, but for most people the straightforward yearly estimate is close enough to be useful. The point is to know whether the interest you are earning is worth keeping your money in that particular account.

Why interest rates change and what that means for your savings

Banks set their savings account interest rates based on what the Federal Reserve does with its benchmark interest rate. When the Federal Reserve raises rates, banks usually raise the rates they offer on savings accounts within weeks or months. When the Federal Reserve lowers rates, banks lower savings account rates too, sometimes when ready.

This means the APY you see today may not be the APY you earn six months from now. If rates are falling, your interest earnings will shrink. If rates are rising, your earnings will grow. You can switch to a different bank if your current bank's rate falls too far behind, but you will have to move your money and open a new account.

Some banks offer promotional rates that are higher than their standard rates, but only for a limited time or only on new deposits. Read the fine print to understand when the promotional rate ends and what your rate will be after that.

The difference between savings accounts and money market accounts

Money market accounts are a hybrid between savings accounts and checking accounts. They often pay higher interest rates than regular savings accounts, but they usually come with limits on how many withdrawals you can make per month and may require a higher minimum balance.

If you need to access your money frequently, a regular savings account is simpler. If you have a larger balance and do not need to withdraw often, a money market account might earn you more interest. The interest calculation works the same way in both — daily calculation, monthly crediting, APY comparison — so you use the same math to figure out what you will earn.

Frequently Asked Questions

Does the interest rate change if I withdraw money mid-month?

The interest you earn that month is based on your balance during the month, not what you have at the end. If you use the average daily balance method, withdrawing money lowers your average and reduces that month's interest. If your bank uses the lowest balance method, one withdrawal can lower your interest for the entire month.

Why do online banks pay more interest than banks with physical branches?

Online banks have lower operating costs because they do not maintain buildings and staff branches. They pass some of those savings to customers by offering higher interest rates. The money is equally safe — online banks are insured by the FDIC just like traditional banks — so there is no reason not to compare online accounts when shopping for the best rate.

Can I lose money if the interest rate drops?

No. Interest rates can only go down to zero; they cannot go negative on savings accounts. If rates drop, you straightforward earn less interest going forward, but the money you already have stays in your account. You will not lose any of your principal balance.

What happens to my interest if I close the account before the end of the month?

You receive the interest that was earned up to the day you close the account. The bank calculates it based on your balance during the time you held the account, then adds it to your final withdrawal. You do not lose any interest by closing early.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on savings accounts is considered income and must be reported on your tax return. Banks send you a form called a 1099-INT if you earn $10 or more in interest during the year. Keep records of your interest earnings for tax time.