The math behind what your bank pays you

Your savings account interest is calculated by multiplying your balance by the annual percentage yield (APY), then dividing by the number of days in a year. Most banks compound interest daily or monthly, meaning they add earned interest back into your account, and then calculate the next period's interest on that larger amount. The result: you earn interest on your interest.

The basic formula is: Interest earned = (Balance × APY) ÷ 365 for one day. Banks repeat this calculation every day (or every month, depending on their compounding schedule), and the interest compounds—each day's calculation includes yesterday's earned interest.

You do not need to do this math yourself. Your bank calculates it automatically and deposits the interest into your account on a schedule they set (usually monthly). But understanding how it works helps you compare accounts and predict what you will actually earn.

Key Takeaways

  • Interest is calculated by multiplying your balance by the APY and dividing by 365 (or 360, depending on the bank's method).
  • Compounding means interest is added to your balance, and the next calculation includes that interest—so you earn more over time.
  • Daily compounding earns slightly more than monthly compounding because interest accrues more frequently.
  • Your bank does the calculation for you, but you can estimate your earnings by dividing the APY by 12 and multiplying by your average balance.
  • The actual interest you receive depends on whether your balance stays constant, grows, or shrinks during the month.

Simple interest vs. compound interest

Simple interest is calculated once on your original balance and never changes. If you deposit $10,000 at 4% APY for one year with simple interest, you earn $400 and end with $10,400. The $400 never earns interest itself.

Most savings accounts use compound interest, which recalculates periodically and includes previously earned interest in the new calculation. With the same $10,000 at 4% APY compounded daily, you earn slightly more than $400 because each day's interest gets added to the balance before the next day's calculation begins. Over a year, the difference is small but real—roughly $408 instead of $400.

The more frequently interest compounds, the more you earn. Daily compounding beats monthly compounding, which beats quarterly. However, the difference shrinks as the APY gets smaller. At 0.5% APY, daily vs. monthly compounding might earn you a few cents more per year on a $10,000 balance.

How to estimate your monthly interest earnings

If you want a rough estimate without doing daily calculations, divide the APY by 12 and multiply by your average balance for the month. For example: a $25,000 balance at 4.5% APY would earn roughly $93.75 per month ($25,000 × 0.045 ÷ 12). This method assumes your balance stays constant, so it works best if you are not regularly depositing or withdrawing.

If your balance changes during the month, use your average balance instead. Add up your balance on several days throughout the month and divide by the number of days you checked. Then apply the formula above. Many banks calculate interest based on the average daily balance, which is why your actual earnings may differ slightly from a single-day estimate.

Your bank's website or app usually shows the interest earned in your account history. Check there first—it is the most accurate number, because it reflects exactly how your bank compounds interest and handles balance changes.

Why different banks show different earnings on the same APY

Two banks offering 4% APY may pay you slightly different amounts because of how they compound interest. A bank that compounds daily will pay more than one that compounds monthly, even at the same rate. Some banks also use 360 days instead of 365 in their calculation, which changes the result slightly.

The difference is usually small—a few dollars per year on a typical balance—but it adds up over time. When comparing savings accounts, look at the APY first, then ask how often the bank compounds interest. Daily compounding is the standard at most online banks.

Some banks also offer promotional rates that apply only to new deposits or for a limited time. Read the terms carefully: the higher rate might expire after three months, or it might apply only to balances above a certain amount. Once the promotion ends, your rate drops to the standard rate for that account type.

What happens to interest if you withdraw money mid-month

If you withdraw money before the month ends, your interest for that month is recalculated based on your actual balance during the period you held the money. You do not lose all the interest you earned—you lose only the interest that would have been earned on the withdrawn amount for the remaining days.

For example: if you have $10,000 on the first of the month and withdraw $5,000 on the 15th, you earn interest on $10,000 for 14 days and on $5,000 for the remaining 16 days. Your bank calculates this automatically; you simply receive less interest that month than you would have if you kept the full balance.

Some savings accounts have minimum balance requirements. If your balance drops below the minimum, the bank may charge a fee or reduce your APY for that month. Check your account terms to see if this applies to you.

How APY differs from interest rate

The interest rate (also called the nominal rate) is the percentage your bank quotes before compounding. The APY (annual percentage yield) is the rate you actually earn after compounding is factored in. APY is always equal to or higher than the interest rate, because compounding adds extra earnings.

Banks are required to display the APY prominently, so that is the number to use when comparing accounts. If one bank shows a 4% interest rate compounded daily and another shows 4% APY, the first bank is being less transparent—they are hiding the fact that daily compounding will earn you slightly more than 4%.

When you see an APY advertised, that is the actual yield you will receive, assuming your balance and the rate stay constant for a full year.

Tracking your interest over time

Most banks show interest deposits in your transaction history. Check your account monthly to see how much you earned. Over several months, you will notice the amount changes slightly based on your balance and whether the bank adjusted the APY.

If you want to track earnings across multiple accounts, create a simple spreadsheet: list each account, its APY, your average balance, and the interest earned each month. This helps you see which accounts are performing well and whether it makes sense to move money to a higher-yielding account.

Keep in mind that APY changes. Your bank can lower the rate at any time, though they usually give you notice. If rates drop significantly, you may want to shop around for a better account. Conversely, if rates rise, your earnings will increase automatically without any action on your part.

Frequently Asked Questions

Can I calculate interest if my balance changes every day?

Yes, but it is tedious. You would multiply each day's balance by the daily rate (APY ÷ 365), then add all those daily earnings together. Your bank does this automatically, so check your account history for the actual amount. If you want an estimate, use your average daily balance for the month instead.

Does interest compound on interest I have already earned?

Yes, that is what compounding means. Once interest is added to your account, the next compounding period includes that interest in the calculation. This is why compound interest earns more than simple interest over time, even at the same rate.

What if my bank changes the APY mid-year?

Your interest for that month is calculated using the rate that was in effect during the period you held the money. If the rate changed on the 15th, you earn interest at the old rate for the first 14 days and the new rate for the remaining days. Your bank handles this calculation automatically.

Is the interest I earn on a savings account taxable?

Yes. Interest income is taxable as ordinary income at the federal level and in most states. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. Some states do not tax savings account interest, so check your state's rules.

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

Check your account history and compare the interest shown to your balance. If you earned roughly 1/12 of the APY multiplied by your average balance, the calculation is correct. If the amount is significantly lower, contact your bank and ask how they calculated it. Errors are rare, but it is worth verifying if something looks off.