The basic formula: your balance, the rate, and the time period

Banks calculate savings account interest by multiplying three things: the money you have in the account, the interest rate the bank is paying, and how long that money sits there. The formula is Interest = Balance × Annual Rate ÷ 365 × Number of Days. If you have $1,000 in an account earning 4.5% annually and leave it there for 30 days, you earn roughly $3.70 before the bank rounds down.

The catch is that most banks do not calculate interest once a year. They calculate it daily or monthly, then add it to your account. This matters because once interest is added, the next calculation includes that interest too — a process called compounding. A $1,000 account earning 4.5% compounded daily will earn slightly more than one where interest is calculated once at the end of the year.

The exact amount you earn depends on three things the bank controls: the stated annual percentage rate (APR), how often they compound, and whether they use your average daily balance or your ending balance. You can find the APR in your account agreement or on the bank's website. The compounding frequency is usually daily or monthly. The balance method varies — some banks average what you had each day of the month, others use only your final balance on the last day.

Key Takeaways

  • Interest is calculated by multiplying your account balance by the annual rate and dividing by the number of days in the year, then multiplying by how many days your money was in the account.
  • Most banks compound interest daily or monthly, meaning interest earned gets added to your balance and then earns interest itself in the next period.
  • The annual percentage rate (APR) is what matters for comparing accounts — it already accounts for how often the bank compounds.
  • Your actual earnings depend on whether the bank uses your average daily balance or your ending balance, which you can find in the account disclosure.

Why daily compounding earns more than monthly or annual

Compounding is the reason a savings account earning 4.5% does not simply earn $45 per year on $1,000. Instead, it earns slightly more because interest gets added to your balance partway through the year, and that added interest earns interest too.

Here is the difference in real numbers. A $1,000 account at 4.5% compounded annually earns $45 in year one. The same account compounded monthly earns about $45.84. Compounded daily, it earns about $46.09. The difference grows as your balance grows and as time passes. After five years, daily compounding will have earned you roughly $25 more than annual compounding on that same $1,000 — not huge, but real.

Banks advertise the APY (annual percentage yield) to show you the real earnings rate after compounding is factored in. The APY is always higher than the APR when compounding happens more than once a year. If a bank shows you a 4.5% APY, that number already includes the effect of daily or monthly compounding, so you can compare it directly to another bank's APY without doing math yourself.

Average daily balance versus ending balance

Some banks calculate interest on your average daily balance — they add up what you had each day of the month and divide by the number of days. Others use your ending balance on the last day of the month. This can change how much you earn, especially if your balance moves around.

Imagine you have $5,000 in your account on the first day of a 30-day month, then withdraw $4,000 on day 15. With average daily balance, the bank counts $5,000 for 14 days and $1,000 for 16 days, giving an average of about $2,467. With ending balance, the bank counts only the $1,000 you had at the end. At 4.5% compounded daily, the average daily balance method earns you roughly $4.56 for the month, while the ending balance method earns about $0.37. The difference is significant.

Your account agreement or the bank's disclosure statement will tell you which method they use. Most online banks and many traditional banks use average daily balance because it is fairer to customers who move money around. Some banks use ending balance, which rewards you only for money you kept there the whole month.

How interest posting dates affect when you see the money

Banks calculate interest continuously, but they post it — actually add it to your account — on a schedule. Most post monthly, some post daily, and a few post quarterly. The posting date is when the interest actually becomes yours and starts earning interest in the next compounding period.

If a bank calculates interest daily but posts it monthly, you earn interest on interest every day, but you do not see the money in your account until the posting date. This does not change how much you earn — the compounding still happens — but it affects when you can withdraw it. Some banks post on the last day of the month, others on the first day of the next month. Check your account agreement or call the bank to find out.

The posting date also matters if you are comparing accounts. A bank that calculates and posts daily will show you the interest sooner, but the total amount you earn over a year is determined by the APY, not by how often they post. Do not choose an account based on posting frequency alone.

What happens when rates change

Banks can change the interest rate on savings accounts at any time, and they often do when the Federal Reserve raises or lowers its benchmark rate. When a rate changes, the new rate applies to interest calculated after the change date, not to interest already posted.

If your account earns 4.5% on January 1 and the bank lowers it to 3.5% on February 1, you earn 4.5% on your January balance and 3.5% on your February balance. The bank must notify you before lowering a rate, usually by email or mail, and the notice will say when the change takes effect. You can then decide whether to move your money to a different bank.

Rate increases happen less often, but when they do, they apply immediately to new interest calculations. You do not have to do anything to benefit from a rate increase — the bank automatically uses the new higher rate on your next interest calculation.

Fees and how they reduce your earnings

Interest earned is reduced by any fees the bank charges. A $1,000 account earning $46 per year loses money if the bank charges a $5 monthly maintenance fee — that is $60 per year, leaving you with a net loss of $14 even though the account is earning interest.

Most online banks and many traditional banks waive monthly fees if you maintain a minimum balance, usually $500 to $2,500. Some charge fees only if your balance drops below a threshold. Others charge no fees at all. Before opening an account, check the fee schedule in the account agreement. A high interest rate does not matter if fees eat up the earnings.

Fees are deducted from your balance, which then reduces the amount earning interest in the next period. This creates a compounding effect in reverse — a fee reduces your balance, which reduces the interest you earn, which reduces your balance further. Over time, fees on a low-balance account can cost more than the interest you earn.

How to find the real rate you will earn

The APY is the number to use when comparing accounts. It is the only rate that accounts for how often the bank compounds interest, so two banks with different compounding schedules can be compared directly by looking at their APYs.

You can find the APY on the bank's website, in the account agreement, or in the disclosure statement the bank sends you when you open the account. It will be listed next to or near the APR. The difference between them tells you how much compounding is adding to your earnings — usually a small amount, but it adds up over time.

To estimate what you will actually earn, multiply your balance by the APY and divide by 12 for a monthly estimate. A $10,000 account at 4.5% APY earns roughly $37.50 per month before fees. Subtract any monthly fees to get your real earnings. If the account has a $5 monthly fee, your real earnings are $32.50 per month.

Frequently Asked Questions

Does my interest earn interest?

Yes, if your bank compounds interest — which almost all do. Interest posted to your account becomes part of your balance and earns interest in the next compounding period. This is called compounding. The more often the bank compounds (daily is better than monthly), the more your interest earns interest.

What is the difference between APR and APY?

APR is the stated annual rate before compounding. APY is the real rate you earn after compounding is included. APY is always higher than APR when compounding happens more than once a year. Use APY to compare accounts because it shows the actual return you will get.

Can a bank change my interest rate without telling me?

No. Banks must notify you before lowering a rate, usually by email or mail at least 30 days before the change takes effect. Rate increases do not require advance notice. You can close the account and move your money if you disagree with a rate decrease.

Why do online banks pay higher interest than traditional banks?

Online banks have lower overhead costs because they do not operate physical branches. They pass some of those savings to customers through higher interest rates. The interest calculation is the same — balance times rate times time — but the rate itself is higher at online banks, so you earn more on the same balance.

What happens to my interest if I withdraw money mid-month?

If your bank uses average daily balance, you earn interest on the money for the days it was in the account. If they use ending balance, you earn nothing on money you withdrew. Check your account agreement to see which method your bank uses, because it affects how much you earn when your balance changes during the month.