Banks multiply your balance by an interest rate and divide by the number of days in a year

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360, depending on the bank), then multiplied by the number of days the money sat in the account.

The result is that you earn interest not just on your original deposit, but also on the interest that was added in previous days. This is called compound interest, and it means your money grows faster than if the bank simply paid you interest once a year on your starting balance.

Different banks compound at different intervals — some daily, some monthly, some quarterly — but daily compounding is now standard for most online savings accounts and high-yield savings accounts. The more frequently interest compounds, the more you earn, though the difference between daily and monthly compounding is usually small unless your balance is very large.

Key Takeaways

  • Daily compounding means the bank calculates interest on your balance every day and adds it back to your account, so you earn interest on your interest.
  • The annual percentage yield (APY) shown by the bank already accounts for compounding, so you do not need to do the math yourself — that is the actual rate you will earn.
  • A higher APY always means more money in your account over time, regardless of how often interest compounds.
  • Interest is only paid on money that actually sits in the account; withdrawals reduce your balance and the interest earned on future days.

The difference between APR and APY

Banks advertise two different rates: APR (annual percentage rate) and APY (annual percentage yield). The APR is the raw interest rate before compounding. The APY is the rate you actually earn after compounding is factored in.

For savings accounts, always look at the APY, not the APR. The APY is what you will actually receive. For example, if a bank offers 4.50% APY, that is the total return you will see on your money over one year, assuming you do not withdraw anything and the rate does not change. The APR might be slightly lower — say 4.48% — but the bank has already done the compounding math and converted it to APY for you.

The difference between APR and APY grows larger as interest rates rise and as balances grow larger, but for most savings accounts the gap is less than 0.1%. Still, when comparing two accounts, always compare APY to APY, never APR to APY.

How your balance affects the interest you earn

Interest is calculated on your actual balance on each day. If you deposit $10,000 on Monday and withdraw $5,000 on Wednesday, the bank calculates interest on $10,000 for Monday and Tuesday, then on $5,000 for Wednesday onward. Your balance changes, so the interest earned each day changes too.

This means the timing of deposits and withdrawals matters. Money that sits in the account for the full month earns more than money that arrives partway through. If you deposit $1,000 on the first of the month, it earns interest for 30 days. If you deposit $1,000 on the 15th, it earns interest for only 15 days, so you receive half as much interest on that deposit.

Some banks use the average daily balance method instead of calculating interest on each day's balance separately. With this method, the bank adds up your balance for each day of the month and divides by the number of days. The result is one average balance, and interest is calculated on that. The outcome is usually very similar to daily compounding, but the math is slightly different.

Why rates change and how that affects your earnings

Banks set their own interest rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise savings account rates within days or weeks. When the Fed cuts rates, banks cut savings account rates just as quickly. Your rate can change at any time, and banks are not required to give you advance notice.

This means the APY you see advertised today may not be the rate you earn next month. If rates fall, your earnings will fall. If rates rise, your earnings will rise. The interest you have already earned stays in your account — only future interest is affected by rate changes.

High-yield savings accounts tend to raise and lower rates faster than traditional bank savings accounts, so if you are watching your rate closely, a high-yield account will usually reflect Fed changes sooner.

How to calculate interest yourself if you want to verify

You do not need to calculate interest yourself — your bank statement will show exactly how much you earned. But if you want to check the math or estimate what you will earn, the formula is: (Balance × APY ÷ 365) × Number of Days = Interest Earned.

For example, if you have $10,000 in an account earning 4.50% APY and you want to know how much interest you will earn in 30 days: ($10,000 × 0.045 ÷ 365) × 30 = $36.99. That is the interest you would earn on that balance for that period, assuming the rate does not change and you do not make any deposits or withdrawals.

If your balance changes during the month, you have to break the calculation into pieces — one calculation for each day's balance — or use the average daily balance method. Most people find it easier to just check their statement after the fact, since the bank has already done the work.

What happens to interest if you close your account early

Interest accrues (builds up) every day, but it is only credited to your account on specific dates. Most banks credit interest monthly, on the last day of the month or the first day of the next month. Some credit it quarterly or even annually, though this is rare for savings accounts.

If you close your account before the interest is credited, you may lose that accrued interest. For example, if your bank credits interest on the last day of the month and you close your account on the 25th, you will not receive the interest that was accruing during those 25 days. Always check your bank's policy before closing an account, and try to close it on or after the interest credit date if you want to keep the interest you have earned.

Frequently Asked Questions

Does interest compound if I do not touch my account?

Yes. Interest compounds automatically every day, whether you log in or not. The bank calculates interest on your balance, adds it to your account, and then calculates interest on the new balance the next day. You do not have to do anything for compounding to happen.

What if my bank uses 360 days instead of 365?

Some banks use 360 days in their interest calculation instead of 365. This results in slightly higher interest paid to you, because you are dividing by a smaller number. The difference is small — roughly 1.4% more interest — but it works in your favor. Check your account agreement to see which method your bank uses.

Can I earn interest on interest that has already been credited?

Yes, that is exactly what compound interest is. Once interest is added to your account, it becomes part of your balance, and the next day's interest calculation includes it. This is why compound interest grows faster than simple interest.

Does a withdrawal stop interest from accruing?

No. Interest accrues on your balance every day, including days when you make withdrawals. If you withdraw money on Wednesday, interest still accrues on the remaining balance for the rest of the month. The interest is calculated on whatever balance you actually have each day.

Why do some banks show different interest rates for different balance tiers?

Some savings accounts offer higher rates on larger balances. For example, balances under $25,000 might earn 4.00% APY, while balances of $25,000 or more earn 4.50% APY. The bank calculates interest separately on each tier of your balance using the rate for that tier. This is less common now, but it still exists at some traditional banks.