Interest is calculated on your balance, compounded at intervals your bank sets

Your bank multiplies your account balance by an interest rate, then divides by the number of times per year interest compounds. That calculation happens on a schedule — daily, monthly, or quarterly — and the result gets added to your account. The more often interest compounds, the more you earn, because you start earning interest on the interest itself.

The actual formula banks use is: Interest = Principal × (Annual Interest Rate ÷ Compounding Periods) × Number of Days in Period ÷ 365. But you do not need to calculate it yourself. Your bank publishes the Annual Percentage Yield (APY), which already accounts for compounding and tells you the real return you will get in a year.

The difference between the stated interest rate and the APY matters. A savings account offering 4.50% APY will earn you more than one offering 4.50% annual interest rate compounded quarterly, because the APY number already includes the compounding effect.

Key Takeaways

  • Banks calculate interest by multiplying your balance by the interest rate, then adding that amount to your account on a schedule they set — usually daily or monthly.
  • Compound interest means you earn interest on the interest already added to your account, so the frequency of compounding directly affects how much you earn.
  • The Annual Percentage Yield (APY) is the number that matters for comparing accounts, because it shows your real yearly return after compounding is factored in.
  • Interest accrues even if you do not withdraw it, and it stays in your account unless you move it or close the account.
  • Your bank must disclose the interest rate, compounding frequency, and APY before you open the account, usually in a document called the Truth in Savings disclosure.

How compounding frequency changes what you earn

A bank that compounds interest daily will pay you more than one that compounds monthly, even if both offer the same stated rate. This is because daily compounding adds interest to your account 365 times per year, and each time interest is added, the next calculation includes that new amount.

Suppose you have $10,000 in an account earning 4.00% annual interest. If the bank compounds daily, it divides 4.00% by 365, then multiplies your balance by that fraction each day. After one day, you have earned roughly $1.10. The next day, interest is calculated on $10,001.10, not $10,000. Over a year, daily compounding at 4.00% yields approximately 4.08% APY. Monthly compounding at the same rate yields approximately 4.07% APY. The difference is small in dollar terms on smaller balances, but it compounds over time and across larger amounts.

Banks choose their compounding frequency and must disclose it. High-yield savings accounts typically compound daily. Traditional savings accounts at brick-and-mortar banks may compound monthly or quarterly. The APY your bank shows you already reflects the compounding frequency they use, so you can compare accounts directly by APY without doing the math yourself.

Why your balance matters more than the rate

Interest is always calculated on the balance in your account on the day the calculation happens. If you deposit $5,000 on the first of the month and the bank compounds interest on the last day of the month, the interest is calculated on whatever balance you have on that last day — which might be $5,000, or less if you withdrew money, or more if you made another deposit.

Some banks use the average daily balance method, which adds up your balance for each day of the month and divides by the number of days. This smooths out the effect of deposits and withdrawals. Others use the daily balance method, calculating interest each day on that day's balance. A few still use the minimum balance method, paying interest only on the lowest balance you held during the period. Your bank discloses which method it uses in the Truth in Savings disclosure you receive before opening the account.

This is why moving money into savings before the compounding date can increase your earnings, and why withdrawing money just before that date reduces them. The timing of deposits and withdrawals relative to the compounding schedule affects how much interest you earn.

Reading the interest rate and APY on your account

Your bank shows you two numbers: the interest rate (sometimes called the annual percentage rate or APR for savings accounts) and the Annual Percentage Yield (APY). The interest rate is the percentage the bank applies to your balance. The APY is what you actually earn after compounding is included.

For example, a high-yield savings account might show 4.50% APY. That APY already includes the effect of daily compounding. The underlying interest rate might be 4.48%, but because interest compounds daily, the effective return is 4.50%. When you compare two savings accounts, always compare the APY numbers, not the interest rates, because APY tells you the real amount you will earn.

Your bank must provide this information in writing before you open the account. For online banks, this is usually a PDF or web page labeled "Truth in Savings Disclosure" or "Account Terms and Conditions". For in-person banks, it is often a printed brochure or a document you sign. The disclosure also states how often interest is compounded and which balance calculation method the bank uses.

How interest appears in your account and when you can use it

Interest is added to your account balance on the compounding date — usually the last day of the month or quarter, depending on your bank. You do not have to do anything to receive it. The bank calculates it, adds it to your account, and it becomes part of your available balance when ready. You can withdraw it, transfer it, or leave it to earn more interest.

Some accounts require a minimum balance to earn interest, or they pay a higher rate if you maintain a certain balance. If your balance falls below the minimum, you may earn no interest that period, or you may be charged a fee that offsets the interest you would have earned. Check your account terms to see if a minimum applies.

Interest is taxable income. Your bank will send you a Form 1099-INT at the end of the year if you earned $10 or more in interest. You report this on your tax return. This is true even if you did not withdraw the interest — earning it counts as income, whether or not you use it.

Why APY changes and how it affects new money

Banks change their APY frequently, sometimes weekly. When the Federal Reserve raises or lowers interest rates, banks adjust the rates they offer on savings accounts. A high-yield savings account that paid 5.00% APY six months ago might now pay 4.50%. A traditional savings account might have dropped from 0.01% to 0.001%.

When your bank lowers the APY, the new rate applies to interest calculated after the change takes effect. Interest already in your account is not recalculated. If you earned $50 in interest at 5.00% APY last month, that $50 stays in your account. Going forward, interest on your balance (including that $50) is calculated at the new, lower rate.

This is why the timing of deposits matters. If you know your bank is about to lower its rate, depositing money before the change means the new money earns the higher rate for at least one compounding period. After the rate drops, new deposits earn the lower rate. Over time, as rates change, different portions of your balance may have earned different rates, but your bank does not track this separately — it straightforward applies the current rate to your current balance.

Comparing savings accounts by APY and compounding

When you are deciding between savings accounts, the APY is the primary number to compare. A high-yield savings account at an online bank might offer 4.50% APY, while a traditional savings account at a local bank might offer 0.01% APY. The difference in earnings on $10,000 over one year is roughly $450 versus $1 — a significant gap.

However, APY is not the only factor. Some accounts have monthly fees that reduce your earnings. Some require a minimum balance. Some limit the number of withdrawals per month. Some offer tiered rates, paying higher APY on larger balances. Read the full account terms, not just the APY, to understand what you are actually getting.

You can also use online calculators to see how much interest you will earn over time at different APYs. These calculators use the same compounding formula banks use, so they give you an accurate picture of what to expect. Most high-yield savings account providers have calculators on their websites.

Frequently Asked Questions

Does interest compound on interest I have already earned?

Yes. Once interest is added to your account, the next compounding calculation includes it. This is called compound interest. If you earn $10 in interest one month, the next month's interest is calculated on your original balance plus that $10. Over time, this creates exponential growth, though the effect is small on savings account balances compared to longer-term investments.

What happens to my interest if I close my account?

Interest accrued up to the day you close the account is yours to keep. If your bank compounds monthly and you close the account on the 15th of the month, you will not earn interest for the rest of that month — only for the days you held the account. Some banks pay accrued interest even if you close before the compounding date; others do not. Check your account terms or ask before closing.

Can a bank change the interest rate on my existing account?

Yes. Banks can change APY at any time, and the new rate applies to interest calculated after the change. You are not locked into the rate you opened the account with. If your bank lowers the rate and you want a higher return, you can move your money to a different bank. There is no penalty for moving savings between banks.

Why is my APY lower than the interest rate shown?

It is not — it should be higher or equal. The APY includes the effect of compounding, so it is always at least as high as the stated interest rate. If you see an APY that is lower than the interest rate, that is an error. Contact your bank to clarify which number is correct.

How do I know if my bank is calculating interest correctly?

You can verify the calculation using the APY and your balance. Multiply your average balance by the APY and divide by 365, then multiply by the number of days in the period. This gives you an estimate of what you should earn. Your actual interest may vary slightly depending on the exact balance calculation method your bank uses, but it should be close. If it is significantly different, contact your bank.