Banks multiply your balance by the interest rate, then divide by the number of days in a year

Most banks use daily compounding to calculate savings account interest. That means the bank looks at your account balance at the end of each day, multiplies it by the annual interest rate, divides by 365 (or 360, depending on the bank), and adds that tiny amount to your account. The next day, interest is calculated on the new, slightly larger balance. This repeats every single day.

The formula banks use is: (Balance × Annual Interest Rate) ÷ 365 = Daily Interest. If you have $10,000 in an account earning 4.5% annual interest, the daily interest is roughly $1.23. That amount posts to your account, and tomorrow's calculation uses $10,001.23 as the starting balance.

The reason this matters is that money sitting in your account longer earns more interest, and interest earned also earns interest — a process called compounding. A $10,000 balance earning 4.5% compounded daily grows to about $10,461 after one year, not $10,450, because of those daily additions stacking up.

Key Takeaways

  • Banks calculate interest daily by multiplying your balance by the annual rate and dividing by 365, then adding that amount to your account each day.
  • Compounding means interest earned on your money also earns interest, so your balance grows faster than the stated annual rate alone would suggest.
  • The exact interest rate your account earns can change at any time, and banks are required to notify you of rate changes before they take effect.
  • Accounts that compound more frequently (daily rather than monthly) earn slightly more money over time, though the difference is small for most balances.

Why the interest rate on your account can change

Savings account interest rates are not locked in. Banks set their own rates based on what the Federal Reserve does with its benchmark interest rate, which changes several times per year. When the Fed raises rates, banks typically raise savings rates too — sometimes quickly, sometimes slowly. When the Fed cuts rates, banks often cut savings rates faster than they raised them.

Your bank must notify you in writing before lowering your interest rate, and the notification must arrive at least 21 days before the change takes effect. Rate increases do not require advance notice, so your rate can go up immediately. Check your account statements or log into your online banking to see your current rate, which is usually listed as APY (annual percentage yield) rather than just "interest rate."

The difference between APR and APY

APY (annual percentage yield) is what you actually earn on a savings account. It includes the effect of compounding — the interest earned on your interest. APR (annual percentage rate) is the raw interest rate before compounding is factored in. Banks are required to show you the APY, not the APR, when advertising savings accounts, because APY is the honest number.

For example, an account might have a 4.4% APR, but because interest compounds daily, the actual APY is 4.5%. That 0.1% difference comes entirely from compounding. The higher the stated rate and the more frequently interest compounds, the larger the gap between APR and APY becomes.

How often interest is added to your account

Interest is calculated daily at nearly all banks, but it is not added to your account every day. Most banks post interest monthly — meaning all the daily interest earned during the month is added to your balance on the same day each month, usually the last day or the first day of the next month. Some banks post quarterly (four times per year) or even annually.

The posting schedule does not change how much interest you earn overall — a monthly posting of 30 days' worth of interest equals the same amount as 30 daily postings. However, the timing can matter if you are moving money between accounts. Interest posted on the last day of the month will not show up in your account until then, so if you transfer money out on the 28th, you will miss that month's interest.

Why high-yield savings accounts earn more

High-yield savings accounts earn more interest than regular savings accounts because banks set the rates independently. Online banks, in particular, often offer higher rates because they have lower overhead costs than brick-and-mortar banks. A regular savings account at a large national bank might earn 0.01% APY, while an online high-yield account earns 4.5% or higher.

The calculation method is identical — daily compounding, posted monthly — but the starting rate is much higher. Over one year, $10,000 in a regular savings account earning 0.01% grows to $10,001. The same $10,000 in a high-yield account earning 4.5% grows to $10,461. The difference comes entirely from the interest rate the bank chose to offer, not from a different calculation method.

What happens to interest if you withdraw money mid-month

Interest is calculated on your balance at the end of each day. If you withdraw money on the 15th of the month, the interest earned from the 1st through the 14th is already locked in — you will not lose it. Interest earned from the 15th onward will be calculated on your new, lower balance. When interest posts at the end of the month, you receive only the interest earned on the balances you actually held each day.

Some older savings accounts use a different method called the "low balance" rule, where interest is calculated on your lowest balance during the month, even if that balance was only there for one day. This is rare now, but it is worth checking your account agreement if you have an old account. Most modern accounts calculate interest on the actual daily balance, which is fairer to customers who make withdrawals.

How to find your account's interest rate and compounding method

Log into your online banking account and look for the account details or account summary page. Your current APY should be listed there. If it is not, call your bank's customer service number — it is on the back of your debit card — and ask for your current APY and how often interest is posted to your account.

You can also find this information in your account agreement, which most banks make available as a PDF download on their website. Search for "savings account agreement" or "deposit account agreement" on your bank's site. The agreement will state the interest rate, the compounding frequency, and the posting schedule. If your bank has changed the rate recently, you should have received a notice in the mail or an email — check your spam folder if you do not see it.

Frequently Asked Questions

Does interest compound if I do not touch my account?

Yes. Interest compounds automatically every day, whether you log in or not. The bank calculates it, adds it to your balance, and uses the new balance for the next day's calculation. You do not have to do anything for compounding to happen.

Can a bank change my interest rate without telling me?

A bank can raise your rate without notice, but it must notify you at least 21 days before lowering your rate. The notification comes by mail or email. If you miss the notice, you can still contact the bank to ask when the change takes effect and whether you can move your money before it happens.

What is the difference between a savings account and a money market account?

Both use daily compounding and earn interest the same way. Money market accounts often pay slightly higher rates and may require a larger opening balance. Money market accounts also come with a debit card or checkbook, while savings accounts typically do not. Interest calculation is identical between the two.

If I move money between my own accounts, does that affect interest?

No. Moving money between your own accounts at the same bank does not change how interest is calculated. Interest is based on the balance in each account at the end of each day. Transfers between your accounts happen instantly (or within one business day) and do not pause or reset interest accrual.

Why does my bank show a different interest rate online than what I was quoted?

Interest rates change frequently, sometimes multiple times per week. The rate you see online now may be different from the rate when you opened your account or when you last checked. Banks update their advertised rates to reflect current market conditions. Your account earns whatever rate was in effect when you opened it, unless the bank notifies you of a change.