The basic formula: your balance times the rate, divided by days in the year
Banks calculate savings account interest by taking the money you have on deposit, multiplying it by the interest rate the bank is offering, and dividing by the number of days in a year. The result is how much interest you earn in a single day. That daily amount gets added to your account on a schedule — usually monthly, sometimes daily or quarterly — depending on what your bank does.
The simplest version: if you have $1,000 in an account earning 4% annual interest, and the bank compounds monthly, you earn roughly $3.33 in the first month (before any deposits or withdrawals change the balance). That $3.33 gets added to your account, and next month the bank calculates interest on $1,003.33 instead of $1,000.
This matters because the timing of when interest is added — called the compounding schedule — changes how much you actually earn over time. A bank that compounds daily will pay you slightly more than one that compounds monthly, even if both offer the same annual rate.
Key Takeaways
- Interest is calculated by multiplying your account balance by the annual interest rate and dividing by 365 (or 360, depending on the bank's method), then adding that daily amount to your account on a schedule.
- The compounding schedule — how often interest is added to your balance — affects your total earnings, with daily compounding paying more than monthly or quarterly.
- Your actual interest rate may be called APY (annual percentage yield) rather than APR, and APY already includes the effect of compounding.
- The balance used for interest calculation is usually the average daily balance over the month, not your balance on a single day.
- Fees, minimum balance requirements, and account inactivity can reduce or eliminate the interest you earn.
Why banks use different day counts: 365 days versus 360
You might notice that some banks divide by 365 and others by 360 when calculating daily interest. This is not a mistake — it is a choice the bank makes, and it affects how much you earn.
Dividing by 360 (called the "ordinary interest" method) makes the daily rate slightly higher, so you earn a tiny bit more each day. Dividing by 365 (the "exact interest" method) is more precise to the actual calendar. The difference is small — on a $1,000 balance at 4% annual interest, you might earn $1 or $2 more per year with the 360-day method — but it adds up if you have a large balance or keep money in the account for years.
Your bank's disclosure documents will tell you which method they use. If you are comparing accounts at different banks, this is one of the smaller factors to consider, but it is worth noticing.
APY versus APR: which number actually matters for savings
Banks are required to show you two different rates: APR (annual percentage rate) and APY (annual percentage yield). For a savings account, APY is the number that matters.
APR is the straightforward interest rate before compounding is taken into account. APY includes the effect of compounding — it shows you what you will actually earn over a year if you leave the money untouched. If a bank offers 4% APR on a savings account that compounds monthly, the APY will be slightly higher, around 4.07%, because you are earning interest on your interest each month.
When you are comparing savings accounts, always look at the APY, not the APR. The APY is the honest number that tells you what your money will actually grow to. Banks must display APY prominently on their website and in account disclosures, usually near the interest rate.
How the average daily balance method works
Most banks do not calculate interest on your balance on a single day. Instead, they use the average daily balance method: they add up what you had in the account each day of the month, then divide by the number of days.
Here is a concrete example. Suppose you have $1,000 in the account for the first 15 days of the month, then deposit $500 on day 16, leaving you with $1,500 for the remaining 15 days. Your average daily balance is ($1,000 × 15 + $1,500 × 15) ÷ 30 = $1,250. The bank calculates interest on $1,250, not on $1,000 or $1,500.
This method is fairer to you than using only the ending balance, because a large deposit late in the month does not sit there earning nothing. It is also fairer than using the opening balance, because a withdrawal early in the month does not penalize you for the whole month. The average daily balance reflects the money you actually had available.
What happens when you withdraw money before interest is credited
If you withdraw money from your savings account before the interest is added to your balance, you lose the interest you would have earned on that withdrawn amount. The bank calculates interest on the balance that was actually there, so pulling money out early reduces what you earn.
This is one reason some people keep a separate emergency fund in a savings account and try not to touch their main savings account. If you know you will need the money within a month or two, the interest you earn will be small anyway — sometimes just a few dollars — so the withdrawal penalty is less of a concern.
Some banks also have rules about how many withdrawals you can make per month before fees kick in. Check your account agreement to see whether there are limits on how often you can take money out.
How fees and minimum balances reduce your actual earnings
The interest rate your bank advertises is only what you earn if you meet certain conditions. Many savings accounts charge a monthly maintenance fee if your balance drops below a minimum — often $100 to $500, depending on the bank. That fee comes directly out of your account and wipes out several months of interest earnings.
Some banks waive the fee if you set up direct deposit, keep a linked checking account open, or maintain a higher balance. Read the account agreement carefully to see what the actual conditions are. A 4% APY account with a $10 monthly fee is worse than a 3.5% APY account with no fees, because the fee will cost you more than the interest difference over a year.
If your account sits inactive for a long time, some banks may also reduce the interest rate or close the account. Check whether your bank has an inactivity policy and what it means for your savings.
Why high-yield savings accounts earn more than traditional savings accounts
A high-yield savings account uses the same interest calculation method as a regular savings account — daily balance, compounding, APY — but the interest rate itself is much higher. A traditional savings account at a large bank might offer 0.01% APY, while a high-yield account at an online bank might offer 4% or 5% APY.
The reason is that online banks have lower overhead costs than brick-and-mortar branches, so they can afford to pay depositors more. They are still insured by the FDIC (Federal Deposit Insurance Corporation) up to $250,000, just like any other bank, so your money is equally safe.
The interest calculation itself works the same way — your balance times the rate, divided by days in the year, compounded on whatever schedule the bank uses. The only difference is the rate is higher. If you are keeping money in a traditional savings account earning less than 1% APY, moving it to a high-yield account could earn you tens or hundreds of dollars more per year with no additional work.
Frequently Asked Questions
Does interest get added to my account every day or every month?
Interest is calculated daily, but it is usually added (credited) to your account monthly. Some banks credit interest quarterly or even annually. Check your account agreement to see the schedule. Even if interest is only credited monthly, the daily calculation means you earn interest on your interest as soon as it is added.
If I withdraw money mid-month, do I lose all the interest for that month?
No. Banks use the average daily balance method, so you earn interest on the money you actually had in the account each day. If you withdraw $500 on day 15, you lose interest only on that $500 for the remaining 15 days, not on your entire balance for the whole month.
Why does my bank show APR and APY when they are almost the same number?
APR is the base interest rate before compounding. APY includes compounding, so it is always equal to or higher than APR. Banks must show both by law so you can see the effect of compounding. For savings accounts, APY is the number to use when comparing accounts.
Can I earn interest on interest?
Yes, that is what compounding does. When interest is added to your account, the next interest calculation includes that interest as part of your balance. Over time, earning interest on your interest makes a real difference, especially with higher rates and longer time periods.
What if my bank changes the interest rate after I open the account?
Banks can change savings account rates at any time without notice. When rates go up, your earnings increase. When rates go down, your earnings decrease. This is why it is worth checking your current rate occasionally and comparing it to what other banks are offering.