The Basic Formula for Daily Interest

Banks calculate daily interest by taking your account balance at the end of each day, multiplying it by the annual interest rate, then dividing by 365 (or sometimes 360, depending on the bank). The result is what you earn that single day. Most banks do this automatically and add up all those daily amounts at the end of each month or quarter to deposit into your account.

The formula looks like this: Daily Interest = (Account Balance × Annual Interest Rate) ÷ 365. If you have $10,000 in the account and the rate is 4.5% per year, you earn ($10,000 × 0.045) ÷ 365 = $1.23 that day. Tomorrow, if your balance is different, the amount changes.

The key detail: the interest is calculated on whatever balance sits in the account at the end of business that day, not the average balance or the starting balance. This means deposits made early in the day earn interest immediately, and withdrawals made late in the day still reduce what you earn that day.

Key Takeaways

  • Daily interest is calculated by multiplying your end-of-day balance by the annual rate and dividing by 365, then the bank adds up all those daily amounts each month or quarter.
  • Your balance changes every day, so the interest you earn each day is different — higher balance days earn more, lower balance days earn less.
  • Most banks use 365 days in the year for this calculation, though some use 360; check your account terms to know which your bank uses.
  • Interest compounds when the bank deposits your earned interest back into the account, so the next day's calculation includes the interest you already earned.

Why Your Daily Balance Matters More Than Your Starting Balance

Because interest is calculated on the balance at the end of each day, not the balance at the start of the month, the timing of deposits and withdrawals changes how much you earn. A $5,000 deposit made on the first of the month earns interest for 30 days. The same deposit made on the 15th earns interest for only 15 days that month.

This is why some people move money into savings accounts early in the month — every extra day the money sits there, it earns a small amount. Over a year, those small daily amounts add up. Conversely, if you withdraw money partway through a month, you lose the interest you would have earned on that money for the rest of the month.

Banks report your interest earnings on a monthly or quarterly statement. You can see exactly how much interest posted each period and verify it against your balance by using the daily interest formula yourself for a few days.

How Compounding Changes the Calculation Over Time

Compounding means the interest you earn gets added back into your account, and then the next day's interest is calculated on the new, larger balance. This is why the same account earns more over a year than simple math would suggest.

If you start with $10,000 at 4.5% annual interest, the first day you earn $1.23. If the bank compounds daily (most do), that $1.23 gets added to your balance, so on day two your balance is $10,001.23, and you earn interest on that slightly larger amount. By the end of a year, compounding means you earn roughly $460 instead of $450 — the extra $10 comes from earning interest on your interest.

The more often interest compounds, the more you earn. Daily compounding beats monthly compounding, which beats quarterly compounding. Nearly all savings accounts now compound daily, so you do not have to hunt for this feature, but it is worth checking your account terms to confirm.

The Difference Between 365-Day and 360-Day Years

Some banks calculate daily interest using 365 days in the year, and some use 360. This is called the day-count convention. The difference is small but real: a 360-day year makes the daily rate slightly higher, so you earn a bit more each day.

Using 4.5% annual interest as an example: with 365 days, the daily rate is 0.045 ÷ 365 = 0.0001233%. With 360 days, it is 0.045 ÷ 360 = 0.0001250%. Over a year on a $10,000 balance, the 360-day method earns about $12.50 more. Your account terms or the fine print on your rate disclosure will tell you which method your bank uses. Most online banks use 365 days.

Calculating Interest Earned Over a Month or Year

To estimate how much interest you will earn over a longer period, you can use a simplified version of the compound interest formula: Final Balance = Starting Balance × (1 + (Annual Rate ÷ 365))^(number of days). This assumes your balance stays the same and interest compounds daily.

For example, $10,000 at 4.5% for 365 days: Final Balance = $10,000 × (1 + (0.045 ÷ 365))^365 = $10,460.45. You earn $460.45 in interest. If your balance changes during the year — because you make deposits or withdrawals — the calculation becomes more complex, and it is easier to let your bank's statement show you the actual amount.

Most banks also provide an Annual Percentage Yield (APY) on their rate disclosures. This is the total return you would earn in a year if you deposited money and left it untouched, accounting for daily compounding. The APY is always slightly higher than the stated annual interest rate because of compounding. Use the APY to compare accounts across different banks — it is the fairest comparison.

What Happens When Interest Rates Change

Savings account rates change based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks usually raise savings rates within days or weeks. When the Fed cuts rates, savings rates fall. Your daily interest calculation adjusts automatically — the bank updates the rate in their system, and the next day's calculation uses the new rate.

If your rate changes mid-month, your interest for that month is split: some days at the old rate, some days at the new rate. The bank calculates each day separately and adds them all up. You will see the total interest posted on your statement, but the statement may not show the rate change explicitly — you have to check your rate history in your account settings or call the bank to see when it changed.

How to Verify Your Bank's Interest Calculation

You can check whether your bank calculated interest correctly by pulling your statement and your daily balances for a few days, then using the daily interest formula yourself. Pick three random days from the month, note the end-of-day balance for each, multiply by the annual rate, divide by 365, and add those three amounts together. Multiply by roughly 10 (since you sampled 3 out of 30 days) to estimate the month's total.

Your estimate will not match exactly — because you only sampled three days and your balance likely changed every day — but it should be close. If your estimate is significantly higher or lower than what the bank posted, contact the bank and ask them to walk you through the calculation for that month. They can show you the daily balances they used and the rate they applied.

Most banks also let you download your transaction history and balance history as a spreadsheet, which makes it easier to spot patterns or verify calculations yourself over longer periods.

Frequently Asked Questions

Does my bank use 365 or 360 days to calculate daily interest?

Check your account terms, rate disclosure, or the bank's website. Most online banks and large national banks use 365 days. Some regional or smaller banks use 360. The difference is small — roughly $12.50 per year on a $10,000 balance at 4.5% — but it is worth knowing.

If I deposit money on the 15th, do I earn interest on it right away?

Yes. The deposit is included in your end-of-day balance on the 15th, so interest is calculated on it starting that day. You earn interest for the remaining days of the month on that deposit.

Why does my interest vary from month to month if my balance stays the same?

Months have different numbers of days — February has 28 or 29, while July has 31. More days means more interest earned. Also, if your rate changed during the month, some days were calculated at the old rate and some at the new rate, which changes the total.

What is the difference between APY and the annual interest rate?

The annual interest rate is the stated rate. The APY is what you actually earn after accounting for daily compounding. APY is always higher and is the number to use when comparing accounts across different banks.

If I withdraw money mid-month, do I lose all the interest I earned that month?

No. Interest is calculated daily, so you keep the interest earned on the days the money was in the account. You only lose the interest you would have earned on that money for the remaining days of the month.