Banks use your daily balance to calculate interest, not your monthly balance

Most banks calculate savings account interest using the daily balance method. This means the bank looks at how much money you have in the account each day, adds up all those daily amounts for the month, divides by the number of days, and then applies your interest rate to that average. You do not earn interest on your highest balance or your lowest balance — you earn it on what you actually held each day.

The interest rate itself comes from the bank's posted Annual Percentage Yield, or APY. This is the rate you see advertised. The bank divides that yearly rate by 365 (or sometimes 360, depending on the bank's method) to get a daily rate, then multiplies that daily rate by your daily balance. Over a month, all those daily interest amounts add up to your monthly interest deposit.

The timing of deposits and withdrawals matters because they change your daily balance immediately. A deposit on the 15th means you earn interest on that money for the remaining days of the month. A withdrawal on the 20th means you stop earning interest on that amount from that day forward. Banks track this automatically — you do not have to do anything — but understanding how it works helps you see why the interest you receive might be different from what you expected.

Key Takeaways

  • Banks calculate interest on your daily balance, not your total deposits or your ending balance, so the timing of money moving in and out affects how much interest you earn.
  • The Annual Percentage Yield (APY) is divided into a daily rate, which is then multiplied by your balance each day and added up over the month.
  • A deposit made mid-month earns interest for only the days remaining in that month, while a withdrawal stops interest from accruing on that amount immediately.
  • Different banks may use slightly different day-count methods (365 days versus 360 days per year), which can create small differences in the interest you receive.

How the daily balance method works step by step

Here is the actual process a bank follows. On each day of the month, the bank records your account balance at the end of business. If you start the month with $5,000 and make no changes for five days, your daily balance is $5,000 for each of those five days. On day six, you deposit $1,000, so your daily balance becomes $6,000 from that point forward. The bank keeps this running record for every single day of the month.

At the end of the month, the bank adds up all 30 or 31 daily balances. If your balance was $5,000 for six days and $6,000 for the remaining 24 days of a 30-day month, the total would be ($5,000 × 6) + ($6,000 × 24) = $174,000. The bank then divides this sum by the number of days in the month: $174,000 ÷ 30 = $5,800. This $5,800 is your average daily balance.

Next, the bank takes your APY and converts it to a daily rate. If your APY is 4.50%, the bank divides 4.50% by 365 to get approximately 0.0123% per day. Then it multiplies your average daily balance by this daily rate and by the number of days in the month. Using the example above: $5,800 × 0.000123 × 30 = approximately $21.37 in interest for that month. This amount is deposited into your account, usually on the last day of the month or the first day of the next month.

Why your interest varies from month to month

Your monthly interest is not the same every month because your balance changes and the number of days in the month changes. February has 28 days (or 29 in a leap year), while months like July and August have 31 days. A higher balance held for more days produces more interest. A lower balance or a balance held for fewer days produces less.

If you keep $10,000 in your account for all of February in a non-leap year, you earn interest on that $10,000 for 28 days. If you keep the same $10,000 for all of July, you earn interest for 31 days. Even with the same APY and the same balance, July's interest will be higher simply because there are more days. The difference is small but real.

Deposits and withdrawals also shift your monthly total. If you deposit $5,000 on the first day of the month, you earn interest on it for the full month. If you deposit the same $5,000 on the last day, you earn interest on it for only one day. The bank does not round or estimate — it counts actual days and actual balances.

The difference between APY and APR on savings accounts

You will see two terms: APY (Annual Percentage Yield) and APR (Annual Percentage Rate). On savings accounts, APY is what matters because it includes compounding. APR does not account for compounding and is rarely used for savings accounts — it is more common for loans and credit cards.

APY tells you the true yearly return when interest is compounded monthly. If a bank advertises 4.50% APY, that means if you held a balance and earned interest every month without touching the account, you would earn 4.50% of your starting balance over the course of a year. The monthly interest gets added to your balance, and the next month you earn interest on the interest you earned the previous month. This compounding effect is already built into the APY number.

When you see the interest rate advertised, it is always the APY. This is the number you use to understand what your account will actually earn. Do not try to divide the APY by 12 to get a monthly rate — the bank already handles that math when it calculates your daily rate from the APY.

How different banks calculate interest slightly differently

Most banks divide the APY by 365 days to get a daily rate. Some banks divide by 360 days instead. This is called the 360-day method or ordinary interest, and it produces slightly higher interest because you are dividing by a smaller number. The difference is small — on a $10,000 balance at 4.50% APY, the 360-day method might earn you a few cents more per year than the 365-day method — but it is a real difference.

Banks are required to disclose which method they use, usually in the account agreement or the Truth in Savings disclosure document. You can find this information by asking your bank directly or by reading the fine print on their website. If you are comparing two banks with the same APY, the one using the 360-day method will pay slightly more interest, though the difference is rarely enough to be a deciding factor.

Some banks also compound interest daily, while others compound monthly. Daily compounding means interest is calculated and added to your balance every single day, so the next day's interest calculation includes the previous day's interest. Monthly compounding means interest is calculated once per month and added all at once. Daily compounding produces more interest over time because you earn interest on interest more frequently, but again, the difference is usually small.

How to estimate your monthly interest before it arrives

You can do a rough calculation yourself if you want to see what to expect. First, find your average daily balance for the month by adding up your balance at the end of each day and dividing by the number of days. If that feels tedious, use a simpler estimate: add your starting balance and your ending balance, divide by two, and you have a rough average.

Next, take your APY and divide by 365. Multiply that daily rate by your average balance and by the number of days in the month. For example, with a $5,000 average balance, 4.50% APY, and 30 days: (4.50% ÷ 365) × $5,000 × 30 = approximately $18.49. This will not be exact — the bank's calculation is more precise because it uses your actual daily balance, not an estimate — but it gives you a ballpark figure.

The reason to do this is not to catch your bank making an error (they rarely do) but to understand whether your interest is tracking with the rate you were promised. If your bank advertises 4.50% APY but you are consistently earning much less, it might be worth asking why or comparing to other banks.

What happens to interest if you close your account mid-month

If you close your savings account before the end of the month, you still receive interest for the days you held the account. The bank calculates your average daily balance up to the day you close, applies the daily rate to that average, and deposits the interest into your account or pays it to you when you close. You do not lose the interest you earned up to that point.

However, if you close your account on, say, the 15th of the month, you earn interest only on your balance from the 1st through the 15th. You do not earn interest for the remaining days of the month because you no longer hold the account. The bank's calculation stops on your closing date.

Frequently Asked Questions

Does the bank round my interest up or down?

Banks typically round interest to the nearest cent. If your calculated interest is $18.496, the bank rounds to $18.50. If it is $18.494, the bank rounds to $18.49. The rounding method is disclosed in your account agreement. Over many months, rounding differences are negligible.

Why did I earn less interest this month than last month if my balance was the same?

The most common reason is that the month had fewer days. February has 28 days (or 29 in leap years), while other months have 30 or 31. Fewer days means less interest, even with an identical balance. A second reason could be that your balance was lower for part of the month due to a withdrawal or transfer.

If I deposit money on the last day of the month, when do I start earning interest on it?

You start earning interest immediately. The deposit counts as part of your daily balance on the day it is made. If you deposit $1,000 on the 30th of a 30-day month, you earn one day of interest on that $1,000 in that month. The full month of interest on that deposit begins in the next month.

Can I earn interest on interest in a savings account?

Yes, if your bank compounds interest daily or monthly. When interest is added to your account, it becomes part of your balance. The next time interest is calculated, your balance is higher, so you earn interest on the previous month's interest. This is called compounding and is included in the APY figure your bank advertises.

What if my bank uses a 360-day year instead of 365 days?

You will earn slightly more interest because the daily rate is calculated by dividing your APY by 360 instead of 365. The difference is small — typically a few dollars per year on a modest balance — but it works in your favor. Your bank must disclose which method it uses in the account agreement.