Interest is calculated on your balance, compounded at intervals your bank sets
Your bank multiplies your account balance by the interest rate, then divides by the number of times per year interest compounds. Most savings accounts compound daily or monthly. The more often interest compounds, the more you earn, because you earn interest on the interest from previous periods.
The actual formula banks use is: A = P(1 + r/n)^(nt), where P is your principal (starting balance), r is the annual interest rate as a decimal, n is how many times per year interest compounds, and t is the number of years. But you do not need to calculate this yourself—your bank does it and shows you the results.
What matters in practice: a 4.50% annual percentage yield (APY) on $10,000 earning daily compounds differently than the same rate compounding monthly. Daily compounding adds slightly more to your balance over a year. Your bank must disclose both the interest rate and the APY so you can compare accounts fairly.
Key Takeaways
- Banks calculate interest by multiplying your balance by the annual rate, then dividing by how many times per year interest compounds (usually daily or monthly).
- Compounding means you earn interest on previous interest, so daily compounding produces slightly more earnings than monthly compounding at the same rate.
- The APY (annual percentage yield) already accounts for compounding, so comparing APYs between accounts tells you which will earn more over a year.
- Interest is usually credited to your account monthly, even if it compounds daily, so you see the full amount once per month.
- Your balance changes the calculation—deposits increase what earns interest, and withdrawals reduce it, often when ready.
How compounding frequency changes what you earn
Compounding is the schedule on which your bank adds earned interest back into your balance so that interest itself starts earning interest. If your account compounds daily, the bank calculates interest 365 times per year. If it compounds monthly, that happens 12 times per year.
At a 4.50% APY, the difference between daily and monthly compounding on $10,000 is roughly $5 to $10 per year—small but real. The bank's disclosure will state the compounding frequency. Most online savings accounts compound daily because it is the most competitive option and costs them nothing extra to offer.
Some accounts compound quarterly (four times per year) or annually (once per year). These are now rare in savings accounts but still appear in some certificates of deposit or older account types. The less frequently interest compounds, the lower your effective earnings, even at the same stated rate.
Why APY matters more than the interest rate alone
The annual percentage yield (APY) is the rate that already includes the effect of compounding. When a bank advertises "4.50% APY," that number already accounts for how often interest compounds. The plain interest rate (sometimes called the nominal rate) does not.
A bank might offer 4.48% interest compounded daily, which equals 4.50% APY. Another might offer 4.50% compounded monthly, which equals roughly 4.59% APY. Comparing the APYs tells you which account will actually earn more money in your pocket over a year. The APY is the number to use when shopping between banks.
Federal law requires banks to disclose the APY prominently so you can compare fairly. If you see only an interest rate without an APY, ask the bank for the APY before opening the account.
How your balance affects interest calculations
Interest is calculated on the balance in your account at the time of calculation. If you deposit $5,000 on the first of the month and the bank compounds daily, that $5,000 earns interest starting when ready. If you withdraw $2,000 on the 15th, the remaining $3,000 earns interest from that point forward.
Some banks use the average daily balance method, which adds up your balance at the end of each day during the month, then divides by the number of days. This smooths out the effect of deposits and withdrawals. Others use the daily balance method, calculating interest on the exact balance each day. The difference is usually small but can matter if you make large deposits or withdrawals mid-month.
Your account disclosure should state which method the bank uses. Most online banks use daily balance compounding because it is simpler to automate and competitive to offer.
When interest is credited and when it starts earning
Interest compounds (is calculated) on a schedule set by the bank—usually daily. But interest is typically credited (actually added to your balance) once per month, usually on the last business day of the month. This means you see the full month's interest hit your account at once, even though it was compounding every day.
Once interest is credited, it becomes part of your principal balance and starts earning interest itself in the next compounding cycle. If your bank credits interest on the 28th of each month, that interest begins compounding on the 29th.
Deposits usually start earning interest the next business day after they are received, though some banks have a one-day delay. Withdrawals typically stop earning interest when ready. Check your account agreement or ask your bank about the exact timing if you are making large deposits or withdrawals near the end of a month.
How interest rates change and what that means for your earnings
Banks change savings account interest rates frequently, sometimes weekly. When the Federal Reserve raises or lowers its benchmark rate, banks adjust their savings rates within days or weeks. Your rate can go up or down, and the bank must notify you of changes before they take effect.
If rates rise, your earnings increase on the same balance. If rates fall, your earnings decrease. This is why the interest rate you see advertised today may not be the rate you earn six months from now. Some banks raise rates quickly when the Fed moves but lower them slowly—shop around if your rate drops significantly.
Your account agreement will state whether your rate is fixed (locked in for a set period) or variable (subject to change). Most savings accounts are variable. Certificates of deposit (CDs) lock in a rate for a specific term, so you know exactly what you will earn.
Understanding the difference between straightforward and compound interest
straightforward interest is calculated only on your original principal—it does not earn interest on interest. Compound interest earns interest on both your principal and the interest you have already earned. All modern savings accounts use compound interest, which is why the compounding frequency matters.
On $10,000 at 4.50% APY, straightforward interest would earn $450 per year, always. Compound interest earns slightly more because each month's interest gets added to the balance and starts earning interest itself. Over one year the difference is small, but over five or ten years it becomes noticeable.
This is why banks advertise APY rather than a straightforward interest rate—the APY shows the real return you will receive, accounting for compounding. It is the number that reflects what you actually earn.
Frequently Asked Questions
Does my interest get taxed?
Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. The interest is taxed at your ordinary income tax rate, not as capital gains.
Why do different banks offer different rates if they all follow the same formula?
Banks set their own rates based on their funding costs, competition, and business strategy. Online banks typically offer higher rates because they have lower overhead than brick-and-mortar branches. Banks also adjust rates based on how much they need deposits at any given time. The formula is the same; the inputs (the rate itself) differ.
If I withdraw money mid-month, do I lose all the interest for that month?
No. Interest is calculated on your balance each day, so you earn interest on the money you had in the account. If you withdraw on the 15th, you keep the interest earned from the 1st through the 15th. You straightforward do not earn interest on the withdrawn amount from the 16th onward.
Can I earn more interest by moving money between accounts?
No. Moving money between your own accounts at the same bank does not increase interest—the money still earns the same rate. Moving money to a different bank with a higher rate will earn more going forward, but you do not earn retroactive interest on the time it was at the lower-rate bank.
What happens to my interest if the bank fails?
Interest earned up to the point of failure is protected by the FDIC (Federal Deposit Insurance Corporation) up to $250,000 per account type per bank. Interest that was calculated but not yet credited is also protected. You will not lose earned interest if an FDIC-insured bank fails.