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

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360, depending on the bank), then multiplied by the number of days the money sat in the account.

The result is that interest compounds — you earn interest on your interest. If you deposit $1,000 and earn $5 in interest during month one, month two's interest calculation includes that $5. The longer money stays in the account, the more this compounding effect adds up, even though the daily amounts are small.

The actual interest rate you receive depends on what the bank decides to offer. Banks are not required to pay any particular rate, and rates change constantly. Some accounts pay nearly nothing; others (usually online banks) pay rates that shift weekly based on what the Federal Reserve does with its benchmark rate.

Key Takeaways

  • Daily compounding means the bank calculates and adds interest to your account every day, so you earn interest on previously earned interest.
  • The daily interest amount is tiny — usually a fraction of a cent — but compounds over months and years into real money.
  • Your bank's stated annual percentage yield (APY) already accounts for compounding, so you do not need to do the math yourself.
  • The interest rate your bank pays you can change at any time and varies widely between banks, so shopping around matters.
  • Money market accounts and certificates of deposit (CDs) use the same daily compounding method but may offer higher rates in exchange for restrictions.

Why the bank uses daily compounding instead of monthly or yearly

Daily compounding is the standard because it benefits both the bank and the account holder — though it benefits the bank more. From your perspective, daily compounding means you earn slightly more than you would if interest were calculated once a month or once a year. The difference is small on small balances, but it adds up.

From the bank's perspective, daily compounding lets them advertise a higher effective rate (the APY) while keeping the underlying annual percentage rate (APR) lower. Both numbers are real; APY just shows what you actually earn after compounding is factored in. Banks advertise the APY because it looks better to customers.

Some banks still calculate interest monthly or quarterly, though this is rare for savings accounts. You will see this more often with older accounts or accounts at smaller institutions. The account disclosure or terms of service will tell you the compounding frequency.

The difference between APR and APY, and why APY is what matters for savings

APR (annual percentage rate) is the raw interest rate before compounding. APY (annual percentage yield) is what you actually earn after compounding is included. For savings accounts, APY is the number you should compare between banks.

Here is a concrete example: a bank might advertise an APR of 4.50% with daily compounding. The actual APY might be 4.60% because of compounding. If you deposit $10,000, you will earn closer to $460 over a year than $450. The difference grows with larger balances and longer time periods.

Banks are required to disclose both numbers, and they must show APY prominently in account disclosures and online. When you are shopping for savings accounts, compare the APY numbers, not the APR. The APY already includes the compounding effect, so it is the true picture of what you will earn.

How the interest rate changes and when banks adjust what they pay you

Your bank can change the interest rate on your savings account at any time, with no advance notice required. Banks typically raise rates when the Federal Reserve raises its benchmark rate, and lower rates when the Fed cuts. The connection is not automatic — some banks move faster than others, and some move slower.

Online banks tend to adjust rates more quickly and more frequently than brick-and-mortar banks. A large national bank might hold its rate steady for weeks or months even after a Fed change, while an online bank might adjust within days. This is one reason online banks often pay higher rates — they compete on rate more aggressively.

You will not receive a notice every time your rate changes, though banks are required to notify you of material changes. In practice, this means you should check your account terms or contact the bank if you want to know the current rate. Many banks show the current rate on their website or in the mobile app.

How much interest you actually earn depends on your balance and how long money stays in the account

The amount of interest you earn is determined by three things: the balance in the account, the interest rate, and the number of days the money sits there. A larger balance earns more. A higher rate earns more. Money that stays in the account longer earns more.

If you deposit $5,000 on the first of the month and withdraw it on the 15th, you earn interest only on those 15 days. If you leave it in for the full month, you earn interest on 30 or 31 days. The daily interest amount is small — on a $5,000 balance at 4.50% APY, you earn roughly $0.62 per day — but it adds up over time.

This is why the compounding effect matters more with larger balances and longer time horizons. A $50,000 balance earning 4.50% APY earns about $6.20 per day. Over a year, that is roughly $2,250. Over five years, compounding means you earn more than $12,000 in interest.

What happens if you withdraw money before the end of a compounding period

With a regular savings account, you can withdraw money at any time without penalty, and you keep all the interest you have earned up to that point. The bank calculates interest daily and adds it to your balance, so the interest is yours once it hits the account.

Certificates of deposit (CDs) work differently — they have a fixed term (like 6 months or 1 year), and withdrawing early usually triggers a penalty. The penalty is typically a certain number of months of interest. A 12-month CD with a 3-month early withdrawal penalty means you lose three months of interest if you take the money out early. The account disclosure will spell out the exact penalty.

Money market accounts fall in between. They usually allow withdrawals without penalty, but they may limit how many withdrawals you can make per month. If you exceed the limit, you might face a fee or the account could be converted to a regular savings account.

How to find the interest rate your bank is currently paying

The easiest way is to log into your online account or mobile app — most banks display the current APY prominently on the account summary page. If you bank in person, you can call the customer service number on your card or visit a branch.

The account disclosure document (sometimes called a "Truth in Savings" disclosure) will also show the rate, though it may be the rate at the time the document was printed. For the most current rate, check online or call.

If you want to compare rates across banks, websites like Bankrate, DepositAccounts, and the FDIC's National Rates and Rate Caps search tool show current rates from many banks. These sites update frequently, though the rates shown may be a day or two behind real time.

Frequently Asked Questions

Does the interest I earn get taxed?

Yes. Interest income is taxable as ordinary income on your federal tax return. Banks send a 1099-INT form if you earn $10 or more in interest during the year. State income tax may also apply depending on where you live. The interest is taxable in the year it is earned, even if you do not withdraw it.

Why do some banks pay almost no interest while others pay 4% or more?

Banks set their own rates based on what they need to attract deposits and what they can earn by lending that money out. Online banks often pay higher rates because they have lower overhead costs and compete primarily on rate. Large national banks may pay less because they have branch networks and brand recognition that attract deposits without high rates.

If I move money between savings accounts, do I lose the interest I earned?

No. Interest that has already been added to your account is yours to keep. When you transfer money to another bank, you take the principal plus all accumulated interest with you. The only exception is if you withdraw early from a CD — then you may lose some interest as a penalty.

Can a bank lower my interest rate without telling me?

Yes, banks can change rates without advance notice. However, they must notify you of material changes under federal law. In practice, this means you should check your rate periodically if you want to stay informed, especially if rates in the market are changing.

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

Both use daily compounding and earn interest the same way. The main difference is that money market accounts often pay slightly higher rates but may limit withdrawals or require a higher minimum balance. The interest calculation method is identical.