Your bank multiplies your balance by a daily rate, then adds that amount to your account each month

Most banks calculate savings account interest daily, but they credit it to your account once a month. Here is how it works: the bank takes your account balance at the end of each day, multiplies it by a tiny fraction of your annual interest rate, and keeps a running total. At the end of the month, they deposit that total into your account as interest.

The reason banks do this daily instead of monthly is that your balance changes constantly. If they only looked at your balance once a month, people who deposited money on the last day would earn almost nothing, while people who kept money in all month would earn much more. Daily calculation is fairer because it counts every dollar for every day it sits in the account.

The interest rate you see advertised—say, 4.50% annually—is called the Annual Percentage Yield, or APY. This is the total you would earn in a year if you never withdrew money and the rate never changed. Banks break this down into a daily rate by dividing the APY by 365 (or sometimes 360, depending on the bank's method).

Key Takeaways

  • Banks calculate interest daily by multiplying your daily balance by a fraction of your annual rate, then add up those daily amounts at the end of the month.
  • The APY shown in advertisements is the yearly rate; your actual monthly interest is roughly one-twelfth of that, minus the effect of daily compounding.
  • Your balance matters: a higher balance or a higher APY both increase the interest you earn each month.
  • Interest rates change over time, so the amount you earn this month may differ from next month if your bank adjusts its APY.

The formula banks use: daily balance times daily rate

Here is the actual math. If your APY is 4.50%, the bank divides that by 365 to get the daily rate: 4.50% ÷ 365 = 0.0123% per day. Then, at the end of each day, the bank multiplies your balance by 0.000123 (which is 0.0123% written as a decimal). That product is your interest earned that day.

Let's use a real example. Say you have $10,000 in the account and the APY is 4.50%. On a day when your balance is exactly $10,000, you earn $10,000 × 0.000123 = $1.23 in interest that day. The bank does this calculation for every single day of the month, then adds all those daily amounts together and deposits the total on the last day of the month.

If your balance changes during the month—because you deposit or withdraw money—the daily rate stays the same, but the amount you earn that day changes. A day when your balance is $5,000 earns half as much interest as a day when it is $10,000. This is why keeping a higher balance throughout the month increases your total monthly interest.

Why your monthly interest is not exactly one-twelfth of the APY

You might think that if the APY is 4.50%, you would earn 4.50% ÷ 12 = 0.375% each month. But that is not quite right, because of something called compounding. When the bank deposits your interest at the end of the month, that interest becomes part of your balance. The next month, you earn interest on your interest.

This effect is small in the short term but adds up over a year. The APY already accounts for this compounding, which is why it is slightly higher than the straightforward monthly rate. For example, with a 4.50% APY and a $10,000 balance, you might earn about $37.50 in the first month. But by the end of the year, you will have earned closer to $461 total, not $450, because each month's interest earns interest in the following months.

You do not need to calculate this yourself. Your bank statement will show you exactly how much interest you earned each month. The statement will also show your APY, so you can check that it has not changed.

How changes in your balance affect monthly interest

The day you deposit money, you start earning interest on it when ready. The day you withdraw money, you stop earning interest on the amount you removed. This is why the timing of deposits and withdrawals matters.

If you deposit $5,000 on the first day of the month, you earn interest on that $5,000 for all 30 or 31 days. If you deposit the same $5,000 on the last day of the month, you earn interest on it for only one day. Over a full year, depositing early rather than late can add up to real money.

Similarly, if you withdraw money early in the month, you lose the interest you would have earned on that money for the rest of the month. Banks do not pro-rate interest or give you a partial month's worth if you withdraw mid-month. The calculation is based on your balance at the end of each day, so withdrawing on day 15 means you earn nothing on that withdrawn amount from day 15 onward.

What happens when your bank changes the interest rate

Banks change their APY frequently, especially when the Federal Reserve changes its benchmark interest rate. When your bank lowers or raises the APY, the new rate applies to interest earned going forward, not to interest you already received.

If your bank drops the APY from 4.50% to 4.00% on the 15th of the month, you earn the higher rate on your balance from the 1st through the 14th, and the lower rate from the 15th through the end of the month. Your statement will show the total interest for the month, calculated at both rates.

You can check your bank's current APY on their website or by calling their customer service line. Some banks publish their rate history, so you can see how much it has changed over time. If you find a bank offering a higher rate, you can move your money, though you will lose any interest you have not yet received from your current bank.

The difference between APY and APR

APY (Annual Percentage Yield) is what savings accounts use. It includes the effect of compounding, so it shows the real amount you will earn in a year. APR (Annual Percentage Rate) is what loans and credit cards use, and it does not include compounding in the same way.

When you are shopping for a savings account, always compare APYs, not APRs. The APY is the honest number that tells you what you will actually earn. Banks must display the APY prominently, usually right next to the interest rate.

How to track your interest earnings

Your bank statement shows the interest you earned each month, usually in a line item labeled "Interest Paid" or "Interest Earned." You can add up these monthly amounts to see your total interest for the year, or you can check your account online—most banks show year-to-date interest in the account summary.

If you want to estimate next month's interest before your statement arrives, multiply your current balance by the daily rate (APY ÷ 365) and then multiply that by the number of days in the month. This will not be exact—because your balance will change—but it gives you a ballpark figure.

Keep your statements for tax purposes. In the United States, if you earn more than $10 in interest in a year, your bank will send you a Form 1099-INT, and you will owe income tax on that interest. Your statement makes it straightforward to verify the amount reported to the IRS.

Frequently Asked Questions

Does interest compound monthly or daily?

Interest is calculated daily but credited monthly. The bank adds up all the daily interest amounts and deposits the total once a month. Once that interest lands in your account, it becomes part of your balance, and you earn interest on it in future months.

Why is my monthly interest different from last month?

Your balance changed, or your bank's APY changed, or both. A higher balance earns more interest. A higher APY also increases your earnings. Some months have more days than others, which slightly affects the total. Check your statement to see the APY and your average balance for the month.

Can I earn interest on interest before the month ends?

No. Interest is credited only once a month, on the last day or the first day of the next month, depending on your bank. Until then, you earn interest only on your original balance and any deposits you made. Once the interest is deposited, you earn interest on it starting the next day.

What if I close my account mid-month?

You will receive interest for the days you held the account that month. The bank calculates it the same way—daily balance times daily rate—and includes it in your final payout. You will not lose interest just because you closed the account early.

Does a higher APY always mean I should switch banks?

Not automatically. Compare the full picture: APY, minimum balance requirements, fees, and how straightforward it is to access your money. A bank with a slightly lower APY but no fees and no minimum balance might earn you more in the long run than a bank with a higher rate and a $25,000 minimum.