Banks multiply your balance by a rate, then divide by the number of days in a year

Savings account interest is calculated using your account balance, the interest rate your bank offers, and the number of days your money sits in the account. The formula banks use is: Daily Balance × Annual Interest Rate ÷ 365 = Interest Earned. Most banks calculate interest daily but credit it to your account monthly, quarterly, or annually depending on the terms of your account.

The rate itself—what your bank calls the Annual Percentage Yield (APY)—is what you see advertised. If your account earns 4.5% APY and you have $10,000 in the account for a full year with no deposits or withdrawals, you would earn roughly $450 in interest. But if your balance changes during the month, the calculation becomes more granular.

Your bank does not calculate interest on your opening balance once and call it done. Instead, it tracks your balance on each day of the month, adds those daily balances together, divides by the number of days in the month, and applies the interest rate to that average. This is called the average daily balance method, and it is the most common approach.

Key Takeaways

  • Banks calculate interest by multiplying your daily balance by the annual interest rate and dividing by 365, then crediting the result to your account on a schedule set by the bank.
  • The Annual Percentage Yield (APY) shown on your account disclosure is the rate you earn, and it already accounts for how often interest is compounded.
  • Most banks use the average daily balance method, which means deposits and withdrawals during the month change how much interest you earn that period.
  • Interest is calculated daily but credited monthly, quarterly, or annually—you do not see the daily earnings until the bank posts them to your account.
  • Moving money into a savings account mid-month means you earn interest only on the days the money actually sits there, not on the full month.

How the average daily balance method works in practice

Say you open a savings account on June 1 with $5,000. On June 15, you deposit $2,000. On June 25, you withdraw $1,500. The bank tracks your balance each day: $5,000 for days 1–14 (14 days), $7,000 for days 15–24 (10 days), and $5,500 for days 25–30 (6 days). It adds these up: (5,000 × 14) + (7,000 × 10) + (5,500 × 6) = 70,000 + 70,000 + 33,000 = 173,000. Then it divides by 30 days to get an average daily balance of $5,766.67.

If the APY is 4.5%, the bank multiplies $5,766.67 × 0.045 ÷ 365 = $0.71 in interest for that day's calculation. The bank repeats this for every day in June, then adds all those daily interest amounts together and credits the total to your account at the end of the month or on the date specified in your account agreement.

This method rewards you for keeping money in the account longer. A deposit that sits for the full month earns more interest than a deposit that arrives on the last day, because it is included in more daily balance calculations.

The difference between APY and interest rate

You may see two different numbers on your account paperwork: the interest rate and the Annual Percentage Yield (APY). The interest rate is the raw percentage your bank pays. The APY is that rate plus the effect of compounding—the process of earning interest on your interest.

If your bank compounds interest monthly, it calculates interest at the end of month one, adds it to your balance, and then calculates month two's interest on the larger balance. Over a year, this compounding effect makes your actual earnings slightly higher than the stated interest rate alone would suggest. The APY reflects this and is always equal to or higher than the interest rate.

For savings accounts, the difference between the two is usually small—often less than 0.1 percentage points—but it adds up over time. Always use the APY when comparing accounts, because it shows what you will actually earn.

How often interest is compounded and credited

Compounding frequency varies by bank. Some compound daily, some weekly, some monthly. The more often interest is compounded, the more you earn, because you earn interest on a larger balance sooner. However, most savings accounts compound daily or monthly, and the difference between the two is usually a few dollars per year on a typical balance.

Crediting frequency is separate from compounding. A bank might compound interest daily but credit it only once per month. This means the interest is calculated every day, but you do not see it in your account balance until the bank posts it—usually on the last day of the month or on a date specified in your account agreement. Some banks credit quarterly (every three months) or annually (once per year).

Check your account disclosure or ask your bank directly when interest is credited. This matters if you are planning to withdraw money, because interest you have earned but not yet seen in your balance is not yet yours to take out.

Why your interest earnings change month to month

If your balance stays the same every day of the month, your interest earnings are predictable. But most people deposit and withdraw money throughout the month, which changes the average daily balance and therefore the interest earned. A large withdrawal early in the month reduces the average daily balance for the entire month, lowering that month's interest. A large deposit early in the month does the opposite.

This is why keeping a higher balance for longer periods earns more interest than keeping a lower balance. A $10,000 deposit that sits for 30 days earns more than a $10,000 deposit that arrives on day 25, even though the final balance is the same.

Your bank statement should show the interest credited each period. If you want to predict what you will earn, multiply your expected average daily balance by the APY and divide by 365, then multiply by the number of days in the period. This gives you a rough estimate, though the exact amount depends on the precise daily balances your bank records.

How to find your account's interest rate and compounding terms

Your interest rate and compounding schedule are listed in the Truth in Savings Disclosure, a document your bank must provide when you open the account. If you did not receive one, ask your bank for it or download it from your online banking portal. The disclosure shows the APY, the interest rate, how often interest is compounded, and when it is credited to your account.

You can also call your bank's customer service line or visit a branch to ask. Banks are required by federal law to disclose this information clearly, so any representative should be able to tell you the exact terms in minutes.

If you are comparing accounts at different banks, always compare the APY, not the interest rate. The APY already accounts for compounding and shows you the true annual return. A bank advertising a higher interest rate but lower APY is actually paying you less.

What happens to interest if you close the account early

If you close your savings account before interest is credited, you forfeit the uncredited interest. For example, if your bank credits interest on the last day of each month and you close the account on the 20th, you lose the interest that would have been credited on the 30th or 31st.

Once interest is credited to your account—meaning it appears in your balance—it is yours to keep, even if you close the account the next day. The key is the date the bank posts the interest, not the date you withdraw the money.

If you are planning to close an account, check when the next interest credit date is and time your closure after that date if possible. The amount of interest you gain by waiting a few days is usually small, but it is still money you have earned.

Frequently Asked Questions

Does interest compound if I do not touch my account?

Yes. Interest compounds on the schedule your bank sets, regardless of whether you make deposits or withdrawals. If your bank compounds daily, interest is calculated and added to your balance every day. If it compounds monthly, interest is added once per month. You do not have to do anything for compounding to happen.

What if my bank changes the interest rate mid-month?

Banks can change rates at any time, but the change typically applies to interest earned after the effective date. If your rate drops on June 15, interest earned June 1–14 is calculated at the old rate, and interest earned June 15–30 is calculated at the new rate. Your bank must notify you of rate changes before they take effect.

Can I calculate my interest earnings myself?

You can estimate it using the formula: Average Daily Balance × APY ÷ 365 × Number of Days. But the exact amount depends on your bank's precise daily balance records and compounding schedule. Your bank statement shows the actual interest credited, which is the authoritative number.

Why is my interest so low if the APY is 4.5%?

The APY is an annual rate. If you earned interest for only one month, you earn roughly one-twelfth of the annual amount. A $10,000 balance at 4.5% APY earns about $37.50 per month, not $450 per month. Also, if your balance fluctuates, your average daily balance may be lower than your ending balance, which reduces earnings.

Do I pay taxes on savings account interest?

Yes. Interest earned on a savings account is taxable income. Your bank sends you a Form 1099-INT at the end of the year showing how much interest you earned, and you report this on your tax return. This is separate from how the interest is calculated—it is a tax matter, not a banking matter.