The basic formula: multiply your balance by the rate and the time
Interest earned is calculated by taking your account balance, multiplying it by the annual percentage yield (APY), and multiplying that by the fraction of the year the money sat in the account. The simplest version is: Balance × APY ÷ 365 × Number of Days = Interest Earned. If you keep $5,000 in an account paying 4.5% APY for one full year, you earn $225. If you keep it there for only 90 days, you earn roughly $55.48.
Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest to your account. The next day, interest is calculated on the new, slightly larger balance. This compounds over time—you earn interest on your interest. The longer your money stays in the account, the more this effect adds up.
Banks are required to disclose the APY, not just the interest rate, because APY already includes the effect of compounding. When you see "4.5% APY" on a savings account, that number already accounts for daily compounding. You do not need to do anything extra to calculate it.
Key Takeaways
- Interest earned equals your balance multiplied by the APY, divided by 365, multiplied by the number of days your money was in the account.
- Daily compounding means the bank adds interest to your account every day, and the next day's interest is calculated on the larger balance.
- APY already includes the effect of compounding, so you do not need to adjust the rate yourself.
- Your actual earnings will vary slightly from month to month if your balance changes, because interest is calculated on the current balance each day.
- Banks must show you the APY in writing; if you see only an interest rate without APY, ask the bank for the APY so you can compare accounts accurately.
How daily compounding changes your earnings over time
With daily compounding, each day's interest is added to your balance before the next day's interest is calculated. This means you earn a tiny bit more than simple interest would give you. The difference is small in the short term but grows the longer your money stays in the account.
Here is a concrete example. Say you have $10,000 at 4.5% APY for one year with daily compounding. On day one, the bank calculates interest: $10,000 × 0.045 ÷ 365 = $1.23. That $1.23 is added to your account. On day two, interest is calculated on $10,001.23, not $10,000. The difference is tiny—about $0.00015—but it repeats every single day. After 365 days, you have earned $460.47 instead of $450. That extra $10.47 came entirely from compounding.
The longer you leave money untouched, the more compounding works in your favor. After five years at the same rate, the difference between simple interest and compounded interest grows to roughly $60. After ten years, it approaches $130. This is why banks advertise daily compounding—it genuinely does earn you more money over time.
What happens when your balance changes during the month
Most people do not keep the same balance in a savings account for an entire year. You deposit money, you withdraw money, your paycheck lands. The bank handles this by calculating interest on your actual balance each day, then adding it up at the end of the month or quarter.
Say you start January with $5,000. On January 15, you deposit $2,000. On January 25, you withdraw $1,000. The bank calculates interest on $5,000 for 14 days, then on $7,000 for 10 days, then on $6,000 for 6 days. Each day's interest is added to your balance. At the end of January, all those daily interest amounts are totaled and posted to your account as a single deposit.
You do not need to track this yourself. Your bank statement will show the total interest posted for the month. But if you want to verify the calculation, you can use the daily formula for each period when your balance was constant, then add those amounts together.
How to find your APY and verify the bank's calculation
Your bank must disclose the APY in writing before you open an account. Look for it on the account disclosure document, the website, or the savings account comparison table. The APY is always expressed as a percentage and always includes compounding. Do not confuse it with the interest rate, which is lower and does not include compounding.
Once you have the APY, you can spot-check your bank's work. Take your ending balance for the month, subtract any deposits you made that month, and add back any withdrawals. That gives you your starting balance. Multiply the starting balance by the APY, divide by 12 (for one month), and you should get roughly the interest posted. The number will not be exact because the bank calculates daily, not monthly, but it should be very close.
If the interest posted is significantly lower than this rough calculation, contact your bank. It is rare for banks to make errors, but it happens. More often, the issue is that the APY changed during the month—many banks adjust rates weekly or daily based on market conditions. Your statement should show which rate was in effect on which dates.
Why different banks pay different amounts on the same balance
Two banks might both offer savings accounts, but one pays 4.5% APY and another pays 2.0% APY. On a $10,000 balance for one year, the difference is $250 in your pocket. This difference exists because banks set their own rates based on how much they need deposits and what they can earn by lending that money out.
Online banks typically pay higher APY than brick-and-mortar banks because they have lower overhead costs. A bank with no physical branches does not pay rent, utilities, or teller salaries, so it can afford to pay you more. Credit unions sometimes pay higher rates to members as well. The trade-off is usually convenience—you cannot walk into a branch and talk to someone in person.
APY also changes over time. When the Federal Reserve raises interest rates, banks raise the APY they offer on savings accounts. When rates fall, so does APY. Some banks may provide a rate for a set period (like a certificate of deposit), while others change the rate whenever they want. Always check the current APY before opening an account, because the rate you see today may not be the rate you earn next month.
The difference between simple interest and compound interest in practice
Simple interest is calculated once, on your original balance, for the entire period. Compound interest is calculated repeatedly, on a growing balance. For savings accounts, the bank always uses compound interest, usually daily. This is why the APY (which includes compounding) is always higher than the stated interest rate (which does not).
The longer your money stays in the account, the more compound interest outpaces simple interest. For short periods—a few months—the difference is negligible. For years, it becomes meaningful. On $50,000 at 4.5% APY for five years, daily compounding earns you roughly $300 more than simple interest would. That is not life-changing money, but it is real money that you would not earn otherwise.
This is also why moving money between accounts can cost you. If you withdraw $10,000 from a savings account and move it to a checking account that pays no interest, you stop earning that daily compounding. Even for a few weeks, you lose money. This is a small cost, but it adds up if you move money frequently.
How to estimate your earnings without doing the math
If you do not want to calculate interest by hand, your bank's website usually shows you an estimate. Most online banks have a savings calculator where you enter your balance, the APY, and how long you plan to keep the money. The calculator shows you the projected interest earned, including compounding.
You can also use the "rule of 72" as a rough mental shortcut. Divide 72 by the APY to find out roughly how many years it takes for your money to double. At 4.5% APY, 72 ÷ 4.5 = 16 years. This is not exact, but it gives you a sense of how fast your money grows. At 2% APY, it takes 36 years. At 5% APY, it takes about 14 years.
Your bank statement is the most reliable source. It shows the exact interest posted for the month, which already accounts for daily compounding, changing balances, and the current APY. You do not need to verify every deposit—just check that interest was posted and that the amount seems reasonable given your balance and the advertised rate.
Frequently Asked Questions
Does interest get added to my account every day or once a month?
Interest is calculated every day, but it is usually posted to your account once a month or once a quarter. You see one lump-sum deposit on your statement, but that deposit represents all the daily interest that accumulated. Some banks post interest more frequently; check your account disclosure to see when yours posts.
If I withdraw money mid-month, do I lose all the interest I earned?
No. Interest is calculated on your balance each day. If you withdraw money on day 15, you still earn interest on the balance from days 1 through 14. You just do not earn interest on the money after you withdraw it. The interest you already earned stays in your account.
Why does my bank show a different APY on different days?
Banks change APY based on market conditions and how much they need deposits. Some banks adjust rates daily or weekly. If you see the APY change, it applies to new interest going forward, not to interest already posted. Your statement will show which rate was in effect during which period.
Can I calculate interest if the APY changes mid-year?
Yes, but you have to break it into periods. Calculate interest for the days when the first rate was in effect, then calculate interest for the days when the second rate was in effect, then add them together. Your bank does this automatically, so your statement is the easiest way to see the total.
Is the interest I earn on a savings account taxable?
Yes. Interest earned on a savings account is considered income and is taxable. Banks 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. This is separate from calculating how much interest you earned—it just means you owe taxes on it.