Banks multiply your balance by a daily rate, compound it each day, and pay you the total once a month
Your savings account interest is calculated daily, not monthly. The bank takes your account balance at the end of each day, multiplies it by a fraction of the annual interest rate, and adds that amount to your account. It does this every single day. At the end of the month, the bank totals up all those daily additions and deposits the sum into your account as a single payment. This process is called daily compounding—you earn interest on your interest from the day before.
The actual formula is straightforward: the bank divides the annual rate by 365 (or sometimes 360, depending on the bank), multiplies that daily rate by your balance, and repeats this for each day of the month. If your balance changes—because you deposit or withdraw money—the daily calculation adjusts when ready. A higher balance on day 5 means you earn more interest on day 5 than you did on day 4.
Key Takeaways
- Interest is calculated on your account balance every single day, using the annual rate divided by 365 days.
- Each day's interest is added to your account, so you earn interest on yesterday's interest starting tomorrow.
- The bank deposits the total of all daily interest into your account once a month, usually on the same date each month.
- Your balance on any given day determines how much interest you earn that day, so deposits and withdrawals change your monthly total.
- The stated annual percentage yield (APY) already accounts for daily compounding, so it shows the actual return you will receive over a year.
The daily rate: how the annual percentage yield becomes a daily number
Banks advertise an annual percentage yield, or APY. This is the rate you would earn if you left your money untouched for a full year. To calculate daily interest, the bank converts this annual rate into a daily rate by dividing by 365 (some banks use 360, which is slightly more generous to you, but 365 is standard).
If your account earns 4.50% APY, the daily rate is 4.50% ÷ 365 = 0.01233% per day. On a balance of $10,000, that is $10,000 × 0.0001233 = $1.23 in interest for that day. Tomorrow, if your balance is still $10,000, you earn another $1.23. The day after, you earn $1.23 plus interest on the $1.23 you earned yesterday—that is compounding.
The APY figure you see advertised already includes the effect of daily compounding. It is not a straightforward annual rate; it is the actual return you will receive if you hold the account for a year. This is why APY is more useful than the raw interest rate when comparing accounts.
How deposits and withdrawals change your daily interest
Your balance changes throughout the month as you deposit paychecks, move money out, or make purchases. The bank recalculates your daily interest based on your actual balance each day. If you deposit $5,000 on the 15th, your interest calculation jumps up starting on the 16th because your balance is now higher.
This means the timing of deposits matters. Money deposited on the 1st of the month earns interest for 30 or 31 days. Money deposited on the 28th earns interest for only 3 or 4 days. Over a year, depositing early rather than late can add up to real money, especially at higher rates.
Withdrawals work the same way in reverse. If you withdraw $5,000 on the 15th, your daily interest calculation drops starting on the 16th. The bank does not penalize you for withdrawals—savings accounts have no withdrawal limits or fees—but you do earn less interest on money that is not in the account.
When the bank actually deposits your interest into your account
The bank calculates interest daily throughout the month, but it does not deposit it daily. Instead, it totals up all the daily interest earned during the month and deposits the sum once a month, usually on the same date each month. Some banks deposit on the last day of the month; others deposit on the first day of the following month. Check your account statements or your bank's website to see when yours deposits.
Once the interest is deposited, it becomes part of your balance and starts earning interest itself the next day. This is why compounding accelerates over time—each month, your balance grows slightly, and the next month's interest is calculated on that larger balance.
Why different banks show different interest amounts even at the same APY
Two banks offering 4.50% APY may deposit slightly different amounts of interest into your account, and there are a few reasons why. The most common is the number of days used in the calculation: some banks use 365, others use 360. Using 360 days gives you a slightly higher daily rate and slightly more interest. The difference is small—on $10,000 at 4.50%, it amounts to about $1.23 per year—but it is real.
Another reason is the timing of when the bank records your balance. Some banks use the balance at the end of the day; others use an average balance across the month. Most use the end-of-day method, which is simpler and more transparent. Your account statement should show the interest deposited and the method used, though many statements do not explain the calculation in detail.
The APY is standardized by federal regulation, so if two banks state the same APY, you will earn approximately the same amount over a year. Small differences in monthly deposits are normal and usually not worth switching banks over.
How to estimate your monthly interest before it arrives
You can estimate your monthly interest using a straightforward formula: multiply your average balance by the daily rate, then multiply by the number of days in the month. If your balance stays at $10,000 all month and the daily rate is 0.01233%, your estimated interest is $10,000 × 0.0001233 × 30 = $36.99 for a 30-day month.
In reality, your balance probably changes during the month, so your actual interest will differ. A more accurate estimate is to add up your balance at the end of each day, divide by the number of days, and use that as your average balance. Then explore the formula above. Most people find this too tedious and straightforward check their account statement after the interest deposits to see what they actually earned.
If you want to compare accounts before opening one, use the APY to project your annual interest. Multiply your expected balance by the APY and divide by 12 to get a rough monthly figure. This is close enough for comparison purposes, though your actual monthly deposits will vary.
Frequently Asked Questions
Does the interest rate change during the month?
The bank can change the rate at any time, but most changes take effect on the first day of the next month or on a date the bank announces in advance. If the rate changes mid-month, the bank calculates interest at the old rate for the days before the change and the new rate for the days after. Your statement will show the rate used.
What happens to my interest if I withdraw money before the end of the month?
You keep all the interest you earned up to the day you withdrew the money. The bank does not claw back interest or charge a fee. Your interest calculation straightforward adjusts downward for the remaining days of the month because your balance is lower.
Why is my monthly interest different from what I calculated?
The most common reasons are a balance change you forgot about, a rate change mid-month, or a difference in how the bank counts days (365 versus 360). Check your statement for the exact rate used and the dates it applied. Some banks also round interest to the nearest cent, which can account for small differences.
Does compound interest mean my money grows faster over time?
Yes, but the effect is small in a savings account. Compounding means you earn interest on your interest, so each month your balance grows slightly faster than the month before. Over a year, this adds a small percentage to your total earnings. The higher the rate and the larger your balance, the more noticeable the effect becomes.
Can I move my interest to a different account?
Yes. Once the interest is deposited into your savings account, it is your money and you can transfer it, withdraw it, or leave it there to compound further. There is no restriction on what you do with interest once it arrives.