The Basic Formula for Savings Account Interest

Interest earned on a savings account is calculated by multiplying your account balance by the annual percentage yield (APY) and dividing by the number of days in a year, then multiplying by the number of days your money actually sat in the account. The formula is: Interest = (Balance × APY ÷ 365) × Number of Days. Most banks use daily compounding, which means they calculate interest on your balance each day and add it back to your account, so tomorrow's interest calculation includes today's interest.

The reason this matters is that your interest grows faster with compounding than it would if the bank straightforward paid you once a year. A $10,000 balance at 4.5% APY does not earn $450 in one lump sum; instead, the bank calculates roughly $1.23 per day, adds it to your balance, and then calculates interest on that slightly larger balance the next day.

Key Takeaways

  • Daily compounding means the bank calculates interest on your balance each day and adds it back, so your interest earns interest.
  • The formula is (Balance × APY ÷ 365) × Number of Days, but most online calculators or your bank's website will do this for you automatically.
  • Your actual earnings depend on how long money stays in the account and whether you deposit or withdraw during the month.
  • Banks must disclose the APY, not just the interest rate, so you can compare accounts fairly across institutions.
  • Interest is taxable income, and your bank will send you a 1099-INT form if you earned $10 or more in a year.

Why APY Matters More Than Interest Rate

The APY (annual percentage yield) already includes the effect of compounding, while the interest rate does not. A bank might advertise a 4.4% interest rate with daily compounding, but the actual APY would be slightly higher—around 4.5%—because of that compounding effect. When you compare savings accounts, always look at the APY, not the interest rate, because that is the true number that tells you what you will earn.

The difference between rate and yield becomes more noticeable the higher the rate is. At very low rates, the gap is tiny. At 4.5% APY, the difference between the rate and the yield is less than 0.1 percentage points. But this is why banks are required by law to show you the APY prominently—so you can compare fairly.

How to Calculate Interest Manually for a Single Month

If you want to see the math yourself, pick a month and follow these steps. First, find your average daily balance for that month. If your balance was $10,000 for the first 15 days and $12,000 for the remaining 15 days, your average is $11,000. Next, multiply by the APY and divide by 365: ($11,000 × 0.045 ÷ 365) = $1.36 per day. Then multiply by the number of days in the month. For a 30-day month, that is $1.36 × 30 = $40.80 in interest earned.

This is an approximation because banks use the actual daily balance method, not an average. They calculate interest on your exact balance each single day, which means a deposit or withdrawal mid-month changes what you earn. But this method gives you a realistic sense of what to expect.

What Your Bank's Website or App Shows You

Most banks display your interest earnings in your account history or on a monthly statement. You will see a line item labeled "Interest Paid" or "Interest Earned" showing exactly what you made that month. Some banks also show a year-to-date total. If your bank offers an online calculator, you can enter your balance and the APY to see a projection for the next month or year, though the actual amount will vary if your balance changes.

If you want to track interest across multiple accounts or compare what different banks would pay you, a spreadsheet works well. List your balance, the APY, and the number of days, then use the formula above. This is especially useful if you are deciding whether to move money between accounts or open a new one.

How Deposits and Withdrawals Change Your Interest

Every time you deposit or withdraw money, your daily balance changes, which changes the interest calculation. A $5,000 deposit on the 15th of the month means the second half of the month earns interest on a larger balance. A $5,000 withdrawal on the 15th means the second half earns less. Banks calculate this by explore the daily balance method: they look at your balance at the end of each day and use that to calculate that day's interest.

This is why the timing of deposits matters. If you are about to receive a paycheck or a transfer, depositing it early in the month means it earns interest for more days. Conversely, if you need to withdraw money, doing it late in the month means your balance earned interest for most of the month before you took it out.

Interest Earned Versus Interest Paid

Banks calculate interest earned daily but typically pay it monthly or quarterly, depending on the account. Your statement will show the interest paid in that period, which is the sum of all the daily interest calculations. Some accounts pay interest monthly, others quarterly. This does not change how much you earn overall in a year, but it does affect when the money appears in your account and when you can use it.

Once interest is paid into your account, it becomes part of your balance and earns interest itself the next period. This is the compounding effect at work. Over a year, this small difference adds up, which is why even a 0.25% difference in APY can matter on larger balances.

Tax Reporting and Interest Income

Interest earned on a savings account is taxable income. If you earn $10 or more in interest during a calendar year, your bank will send you a Form 1099-INT by January 31 of the following year. You must report this on your tax return. If you have multiple savings accounts, the bank will report the total interest from all accounts you hold with them on a single 1099-INT.

Keep your own records of interest earned, especially if you have accounts at multiple banks. Your tax software will ask you to enter the total interest income, and having documentation makes it easier to verify if there is ever a question. The interest is taxed at your ordinary income tax rate, not at a special rate.

Frequently Asked Questions

Does interest compound daily or monthly?

Most savings accounts compound daily, meaning the bank calculates interest on your balance each day and adds it to your account. Some accounts compound monthly or quarterly, which earns you slightly less because the interest does not earn interest as often. Always check your account terms to see the compounding frequency.

What is the difference between APY and interest rate?

The interest rate is the base percentage the bank pays. The APY includes the effect of compounding, so it is always equal to or higher than the interest rate. Banks must show you the APY so you can compare accounts fairly. Use APY when deciding between accounts.

Can I calculate interest if my balance changes during the month?

Yes, but you need to track your daily balance. Banks use the daily balance method: they calculate interest on your exact balance each day, then add up all those daily amounts. If you deposit or withdraw mid-month, it changes what you earn for the rest of that month.

Will I owe taxes on my savings account interest?

Yes. Interest is taxable income at your ordinary tax rate. If you earn $10 or more in a year, your bank sends you a 1099-INT form. You report this on your tax return. Even small amounts of interest are technically taxable, though the IRS may not require reporting if the total is very low.

How much interest will I earn on $10,000 at 4.5% APY?

Roughly $450 per year if the money stays in the account the whole time and you do not add or withdraw. The exact amount depends on how many days the money is actually in the account. If you deposit it mid-year, you earn less. If you withdraw it mid-year, you earn less for the days after the withdrawal.