Interest is money the bank pays you for keeping your money with them
When you put money in a savings account, the bank uses that money to lend to other customers. In return, the bank pays you interest — a percentage of your balance. The amount you earn depends on three things: how much money you have in the account, what interest rate the bank offers, and how long your money stays there.
Banks calculate interest in different ways. Most savings accounts use something called daily compounding, which means the bank figures out what you've earned each day and adds it back into your account. That new total then earns interest the next day. This creates a snowball effect where your interest earns interest, though the amounts are usually small in a regular savings account.
The interest rate itself changes. Banks set their own rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise what they pay on savings. When the Fed lowers rates, savings rates usually fall too. This is why the rate you see advertised one month might be different from the rate you see the next month.
Key Takeaways
- Interest is calculated daily on most savings accounts by taking your balance, dividing it by 365, multiplying by the annual interest rate, and adding that amount to your account each day.
- Compounding means interest earned one day becomes part of your balance the next day, so you earn interest on your interest, though the effect is small in regular savings accounts.
- The annual percentage yield (APY) shown by banks already includes the effect of compounding, so it is more accurate than the base interest rate alone.
- Your interest earnings depend on the bank's rate, which changes based on Federal Reserve decisions and can vary widely between banks.
The formula banks use for daily interest
Most banks calculate interest using this method: they take your account balance at the end of each day, divide it by 365 (the number of days in a year), multiply that by the annual interest rate, and add the result to your account. This happens every single day.
Here is a concrete example. Say you have $10,000 in a savings account with a 4.5% annual interest rate. The bank divides $10,000 by 365, which equals about $27.40. Then it multiplies $27.40 by 0.045 (the decimal form of 4.5%), which equals about $1.23. That $1.23 gets added to your account on day one. On day two, the bank does the same calculation using your new balance of $10,001.23, which earns slightly more because the balance is slightly higher.
Some banks use 360 days instead of 365 in this calculation, which means you earn slightly more interest. This is rare but worth asking about if you are comparing accounts. The difference is small — on $10,000 at 4.5%, using 360 days instead of 365 would earn you about $1.25 more per year.
What APY means and why it matters more than the interest rate
APY stands for annual percentage yield. It is the total amount of interest you will earn in one year if you do not add or withdraw money, and it includes the effect of compounding. The base interest rate (sometimes called APR in savings accounts, though that term is more common for loans) does not include compounding.
Banks are required to show you the APY, not just the interest rate, because APY tells you the real picture. If a bank offers 4.5% interest with daily compounding, the APY might be 4.60% because of all that compounding adding up over the year. The difference grows larger as the interest rate gets higher, though in most environments the gap is usually less than 0.1%.
When you are comparing savings accounts at different banks, always compare the APY numbers, not the interest rate numbers. The APY is what you will actually earn. Banks list APY prominently on their websites and in account disclosures, usually near the account name or in a rates table.
How your balance affects how much interest you earn
Interest is calculated on your balance, so the more money you have in the account, the more interest you earn. This is straightforward: $10,000 earning 4.5% APY earns roughly twice as much as $5,000 earning the same rate.
What matters for the calculation is your balance at the end of each day. If you deposit $5,000 on Monday and withdraw $2,000 on Wednesday, the bank calculates interest on $5,000 for Monday and Tuesday, then on $3,000 for Wednesday onward. Some banks use the average balance over the month instead, but daily balance is far more common.
This is why keeping money in the account longer earns you more interest. If you leave $10,000 in the account for a full year, you earn interest for 365 days. If you withdraw it after six months, you earn interest for only about 180 days, so you earn roughly half as much.
Why interest rates change and what that means for you
The Federal Reserve, which is the central bank of the United States, sets a target range for a benchmark interest rate called the federal funds rate. Banks use this rate as a reference point when deciding what to pay on savings accounts. When the Fed raises its rate, banks typically raise savings rates within weeks or months. When the Fed lowers its rate, savings rates usually fall.
Banks are not required to match the Fed's moves exactly. Some banks raise savings rates quickly when the Fed goes up, while others move slowly. When the Fed lowers rates, some banks cut savings rates when ready while others wait. This is why you can find very different rates at different banks even when the Fed rate is the same.
If you have money in a savings account, a rising-rate environment is good for you because your earnings will grow. A falling-rate environment means your earnings will shrink. This is one reason some people move money between accounts — to chase higher rates when they appear — though the effort is only worth it if the rate difference is significant (usually 0.5% or more).
The difference between straightforward and compound interest
straightforward interest means the bank calculates interest only on your original balance, not on interest you have already earned. Compound interest means interest is calculated on your balance plus any interest already added to the account.
Savings accounts use compound interest, which is better for you. With straightforward interest on $10,000 at 4.5% for one year, you would earn $450 and end with $10,450. With compound interest calculated daily at the same rate, you earn about $460 and end with $10,460. The difference is small in savings accounts because interest rates are modest and the compounding happens daily rather than monthly or yearly, but it is still in your favor.
Compound interest becomes more powerful over longer periods and at higher rates. If you left $10,000 in the account for 10 years at 4.5% with daily compounding, you would have roughly $15,530. With straightforward interest, you would have only $14,500. That $1,030 difference is the power of compounding working over time.
How to find the interest rate that works for you
Savings account rates vary widely between banks. Online banks typically offer higher rates than brick-and-mortar banks because they have lower overhead costs. Credit unions sometimes offer competitive rates to their members. Large national banks often offer lower rates because they spend more on marketing and branch networks.
To find current rates, visit the websites of banks you are considering and look for the savings account rates table. The rate changes frequently, so a rate you saw last week may not be the same this week. You can also use rate comparison websites, though those sites do not always update when ready and may not include every bank.
When comparing rates, remember that a slightly higher rate on a smaller balance may earn less than a lower rate on a larger balance. A 5% rate on $5,000 earns $250 per year, while a 4% rate on $10,000 earns $400 per year. Also consider the bank's other features: whether it charges monthly fees, whether you can withdraw money without penalty, and whether the bank is insured by the FDIC (which protects your money up to $250,000 if the bank fails).
Frequently Asked Questions
When does the bank add interest to my account?
Interest is calculated daily, but it is usually added to your account monthly. Some banks add it quarterly or even annually. Check your account statement or the bank's disclosure to see when interest posts. You can see the interest earned even before it posts by looking at your account online — many banks show pending interest.
Do I pay taxes on savings account interest?
Yes. Interest earned in a savings account is taxable income. Banks 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 of tax you owe depends on your overall income and tax bracket.
Can I lose money if the interest rate drops?
No. The interest rate is what the bank pays you going forward, not what you have already earned. If rates drop, the interest you already earned stays in your account. You will just earn less interest on future deposits or in future months, but your existing balance does not shrink.
What is the difference between a savings account and a money market account?
Money market accounts often pay slightly higher interest rates than savings accounts, but they usually require a larger minimum balance and limit how many withdrawals you can make per month. Both use daily compounding. Choose based on whether you need frequent access to your money or can leave it untouched to earn more.
Is there a maximum interest rate a bank can offer?
No legal maximum exists. Banks set their own rates based on what they can afford to pay and what they think will attract customers. If a rate seems unusually high, make sure the bank is FDIC-insured and check whether there are hidden fees or balance requirements that reduce the real benefit.