The Basic Formula Banks Use

Banks calculate monthly interest by taking your account balance, multiplying it by the annual interest rate, dividing by 12, and then paying you that amount. The formula is: Monthly Interest = (Account Balance × Annual Interest Rate) ÷ 12.

Here's a concrete example. If you have $5,000 in your account and the bank offers 4.5% annual interest, the math looks like this: ($5,000 × 0.045) ÷ 12 = $18.75 per month. That $18.75 gets added to your account, usually on the same day each month.

The catch is that most banks don't use your full balance for the entire month. Instead, they calculate interest based on your average daily balance — the sum of what you had in the account each day of the month, divided by the number of days. If you deposit $2,000 halfway through the month, that money only earns interest for the remaining days.

Key Takeaways

  • Monthly interest equals your balance times the annual rate, divided by 12, though most banks use your average daily balance instead of your ending balance.
  • The interest rate advertised (like 4.5% APY) is always annual, so you divide by 12 to get the monthly amount.
  • Interest compounds monthly at most banks, meaning next month's interest is calculated on your previous balance plus the interest you just earned.
  • Your bank statement shows exactly how much interest was credited that month, so you can verify the calculation yourself.
  • Different account types and banks use slightly different methods, so the interest you earn can vary even at the same stated rate.

Why Banks Use Average Daily Balance Instead of Your Ending Balance

If banks paid interest on your ending balance, you could deposit money on the last day of the month and earn a full month's worth of interest on just one day's worth of funds. That would cost the bank money. So instead, they track what you had each day and average it out.

Let's say you start the month with $10,000, withdraw $5,000 on day 15, and end with $5,000. Your average daily balance is not $5,000 — it's roughly $7,500 (10 days at $10,000, plus 20 days at $5,000, divided by 30). The interest is calculated on that $7,500, not the $5,000 you ended with.

Your bank statement will show the average daily balance used for that month's calculation. If you want to verify the interest amount yourself, you can use that number in the formula above.

How Compounding Changes Your Monthly Interest

Most savings accounts compound interest monthly, which means the interest you earn gets added to your balance, and next month's interest is calculated on the new, larger balance. This is why the same account earns slightly more each month even if you don't deposit anything new.

In month one, $5,000 at 4.5% APY earns $18.75. In month two, your balance is now $5,018.75, so the interest is calculated on that amount: ($5,018.75 × 0.045) ÷ 12 = $18.82. The difference is small, but over years it adds up. This is the power of compound interest.

Some accounts compound daily instead of monthly, which means interest is added to your balance every single day. Daily compounding earns you slightly more than monthly compounding, though the difference is usually less than a dollar per year on typical balances.

The Difference Between APY and APR

Banks advertise savings account rates as APY (Annual Percentage Yield), not APR. APY includes the effect of compounding, so it's always slightly higher than the actual interest rate the bank uses in its formula. APR does not include compounding.

For savings accounts, this distinction matters less than it does for loans, but it's worth understanding. If a bank shows 4.5% APY, the actual rate used in the monthly calculation might be 4.39%. The difference is small because savings account rates are low, but on very high balances or very high rates, it becomes noticeable.

Your bank statement will show the APY, not the underlying rate. If you want to work backward to find the actual rate used in the formula, you can, but most people just use the APY and accept that the actual monthly interest will be slightly less.

What Happens When Interest Rates Change

Banks can change the interest rate on your savings account at any time, and they often do. When the Federal Reserve raises or lowers its benchmark rate, banks adjust their savings rates within days or weeks. Your next month's interest will be calculated using the new rate.

If rates drop, your monthly interest drops when ready. If rates rise, your monthly interest rises when ready. Some banks are faster to raise rates when the Fed moves up, and slower to lower rates when the Fed moves down — this is normal competitive behavior, not fraud.

You can see the rate history on your account by checking your statements from previous months. Each statement shows the rate that was in effect during that month.

How to Verify Your Bank's Interest Calculation

Pull up your most recent statement and find three numbers: the average daily balance, the interest rate (usually shown as APY), and the interest credited. Then use the formula: (Average Daily Balance × Annual Rate) ÷ 12.

The number you calculate should match the interest credited on your statement, or be within a few cents. If it's off by more than that, contact your bank and ask them to explain the discrepancy. Most banks have made errors before, and they're usually quick to correct them.

If you're comparing accounts at different banks, use the APY to calculate what you'd earn on the same balance at each one. The bank with the highest APY will earn you the most money, assuming you keep the same balance for the same amount of time.

Frequently Asked Questions

Does my interest get taxed?

Yes. Interest earned on a savings account is taxable income. 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. The interest is taxed at your ordinary income tax rate, not at a special rate.

What if I withdraw money mid-month — do I lose that month's interest?

No. Interest is calculated on your average daily balance, so you earn interest on the money for the days you had it. If you withdraw on day 15, you still earn interest on those 15 days' worth of funds. You don't lose the entire month's interest.

Why does my interest seem lower than the advertised rate?

The advertised rate is annual, so you only earn one-twelfth of it each month. Also, if your balance fluctuates, the average daily balance is lower than your ending balance. And if rates dropped during the month, the interest was calculated on the lower rate for part of the month.

Can I calculate interest on a savings account that compounds daily?

The formula is the same, but you'd need to explore it every single day and add the result to your balance before calculating the next day's interest. In practice, your bank does this automatically. You can't easily calculate it by hand, but your statement will show the total interest credited for the month.

What's the difference between a savings account and a money market account for interest purposes?

The calculation method is identical — both use average daily balance and monthly compounding. The difference is that money market accounts often have higher rates but also require larger minimum balances and limit how many withdrawals you can make per month.