Banks use one of two methods to calculate interest: simple interest or compound interest

Most savings accounts use compound interest, which means the bank pays you interest on your original deposit plus all the interest you've already earned. The math compounds — usually daily or monthly — so your balance grows faster than it would with simple interest alone. A few accounts still use simple interest, which pays you interest only on your starting balance, but this is rare for savings accounts.

The bank's calculation starts with your Annual Percentage Yield (APY), which is the rate printed on your account agreement. This is the percentage of your balance the bank will pay you over one year, accounting for how often interest compounds. The bank then divides that annual rate by the number of compounding periods in a year — usually 365 days if interest compounds daily — and applies that smaller rate to your balance each period.

The timing matters. If you deposit money on the 15th of the month and the bank compounds interest daily, you start earning on that deposit the next day. If you withdraw money before the compounding date, you lose the interest that would have been paid on that withdrawn amount.

Key Takeaways

  • Compound interest means you earn interest on your interest, and most savings accounts compound daily or monthly rather than once a year.
  • Your Annual Percentage Yield (APY) is the rate the bank advertises, and it already accounts for how often interest compounds during the year.
  • The bank divides the annual rate by the number of days or months in a year to calculate how much interest you earn each compounding period.
  • Deposits earn interest starting the day after they are received, and withdrawals stop earning interest on the day the money leaves your account.
  • Interest is usually credited to your account monthly or quarterly, even though it may be calculated daily.

How the daily compounding formula works

When a bank compounds interest daily, it uses this basic formula: it takes your account balance at the end of each day, multiplies it by the daily interest rate (the APY divided by 365), and adds that amount to your balance. The next day, it repeats the process using the new, slightly higher balance.

Here is a concrete example. Suppose you have $10,000 in a savings account with a 4.5% APY, and the bank compounds daily. The daily rate is 4.5% ÷ 365 = 0.0123% per day. On day one, the bank calculates $10,000 × 0.000123 = $1.23 in interest. Your new balance is $10,001.23. On day two, the bank calculates $10,001.23 × 0.000123 = $1.23 in interest again (the difference is tiny at this scale). Over a full year, that $10,000 grows to approximately $10,460 — not $10,450, because you earned interest on the interest.

The difference between daily and monthly compounding is small for most balances, but it adds up over time. Daily compounding always produces a slightly higher total than monthly compounding at the same APY, because you earn interest more often.

Why APY is different from the interest rate you might see elsewhere

Banks sometimes list two numbers: an Annual Percentage Rate (APR) and an Annual Percentage Yield (APY). For savings accounts, APY is the number that matters. APY includes the effect of compounding, so it tells you the true amount you will earn in a year. APR does not account for compounding and is usually lower.

For example, a savings account might advertise a 4.4% APR with daily compounding. When you account for the fact that you earn interest on your interest every single day, the actual yield is 4.5% APY. The bank is required to show you the APY on your account agreement and in advertisements, so you can compare accounts fairly.

This is why you should always compare APY between banks, not the interest rate alone. A bank offering 4.5% APY with daily compounding will pay you more than a bank offering 4.5% APR with monthly compounding, even though the numbers look the same.

When interest is actually added to your account

The bank calculates interest daily or monthly, but it does not add the money to your account every single day. Instead, it credits the interest — actually deposits it — on a schedule set by the bank. Most banks credit interest monthly, some credit quarterly, and a few credit daily.

The timing of the credit does not change how much you earn, only when you see the money appear. If your bank credits interest on the last day of the month, you will see a deposit on that date that represents all the interest you earned throughout the month. If you check your balance on the 15th, the interest is still being calculated behind the scenes, but it has not yet been added to your account.

Some banks show you a running total of "interest earned this month" or "pending interest" in your online account view, so you can see what is coming. Others do not show this until the credit date arrives.

How deposits and withdrawals affect your interest calculation

Banks calculate interest based on your daily balance — the amount of money in your account each day. If you deposit $5,000 on the 10th, your balance increases that day, and you start earning interest on the full $5,000 the next day. If you withdraw $2,000 on the 20th, your balance drops that day, and you earn interest only on the remaining amount from that point forward.

This means the timing of deposits and withdrawals within a month changes how much interest you earn that month. Depositing early in the month gives the money more time to earn interest. Withdrawing late in the month means you earn interest on a higher balance for most of the month.

Some banks use an older method called the "average daily balance," which averages your balance across all days in the month rather than calculating interest on each day's balance separately. This method is less common now, but if your bank uses it, the effect is similar: deposits early in the month earn more interest than deposits late in the month.

Why interest rates change and how that affects your earnings

Banks set their own interest rates and can change them at any time. Most savings account rates are tied to the Federal Funds Rate, which the Federal Reserve sets. When the Fed raises its rate, banks usually raise savings account rates within days or weeks. When the Fed lowers its rate, banks usually lower savings account rates just as quickly.

If your bank changes your rate during the month, the interest calculation splits. The bank calculates interest at the old rate for the days before the change, and at the new rate for the days after. You will see the combined total when interest is credited.

This is why shopping for a higher-rate account makes sense. If you move $10,000 from a 0.5% APY account to a 4.5% APY account, you earn an extra $400 per year on that money. Over five years, the difference is $2,000 or more.

How to check if your bank is calculating interest correctly

Your bank statement or online account view should show you the interest credited each month. You can verify this is roughly correct by taking your APY, dividing it by 12, and multiplying by your average balance for the month. This gives you an approximate expected interest payment. The actual amount may be slightly different because the calculation is more precise (it accounts for the exact number of days and daily balances), but it should be close.

For example, if your APY is 4.5%, your monthly rate is roughly 0.375%. If your average balance was $10,000, you should see roughly $37.50 in interest credited. If you see $5 or $50, something is wrong, and you should contact your bank to ask how the interest was calculated.

Keep in mind that if you made deposits or withdrawals during the month, your average balance is lower than your ending balance, so the interest will be lower too. If you are unsure, ask your bank to explain the calculation for a specific month — they are required to provide this information.

Frequently Asked Questions

Does my bank pay interest on money I just deposited?

Yes, but not immediately. Most banks start calculating interest on a deposit the day after it arrives in your account. If you deposit money on Monday, you begin earning interest on Tuesday. The interest is calculated daily but credited (actually added to your account) on the bank's regular schedule, usually monthly.

What happens to my interest if I withdraw money before the end of the month?

You keep all the interest you have already earned up to the day you withdraw. The bank calculates interest on your balance each day, so if you withdraw on the 20th, you earn interest on your full balance through the 19th and on the lower balance from the 20th onward. You do not lose interest you have already earned.

Why does my bank show a different interest rate online than what I remember?

Banks change their rates frequently, especially when the Federal Reserve changes its rate. Your bank may have lowered or raised the rate since you opened the account. Check your most recent account agreement or call the bank to confirm the current rate. The rate that matters for calculating your interest is the one in effect on each day the interest is calculated.

Is compound interest the same at every bank?

The compounding method is similar at most banks — daily or monthly — but the APY varies widely. Two banks that both compound daily may offer very different rates. Always compare the APY, not just the compounding frequency, because a higher APY at one bank will earn you more money regardless of how often interest compounds.

Can I earn interest on interest if my bank compounds monthly instead of daily?

Yes. Monthly compounding still means you earn interest on your interest — it just happens 12 times a year instead of 365 times. The total amount you earn is slightly less than with daily compounding at the same APY, but the difference is small for most balances.