The basic formula for monthly interest
To find out how much interest you earn in a month, multiply your account balance by the annual percentage yield (APY), then divide by 12. The formula is: Monthly Interest = (Account Balance × APY) ÷ 12.
For example, if you have $5,000 in the account and the APY is 4.5%, the math looks like this: ($5,000 × 0.045) ÷ 12 = $18.75 per month. That $18.75 is the interest the bank credits to your account.
This calculation assumes your balance stays the same all month. If you deposit or withdraw money during the month, the interest changes because the bank calculates based on your actual balance each day.
Key Takeaways
- Monthly interest equals your balance multiplied by the APY, then divided by 12.
- Banks calculate interest daily but credit it monthly, so your actual earnings depend on your balance throughout the entire month.
- A higher APY or larger balance produces more interest, but the difference between accounts can be hundreds of dollars per year.
- Compound interest means the interest you earn also earns interest, which is why checking your statement monthly helps you see the effect over time.
Why banks use daily balance calculations
Most savings accounts calculate interest using your daily balance, not your monthly balance. This means the bank tracks what you have in the account each day, multiplies each day's balance by the daily interest rate, and adds all those daily amounts together at the end of the month.
If you deposit $2,000 on the 15th of the month, you earn interest on that $2,000 for only the remaining days. If you withdraw $1,000 on the 20th, your balance drops and so does the interest you earn for the rest of the month. The bank's system handles this automatically—you do not have to calculate each day yourself.
Your monthly statement shows the total interest credited, which is the sum of all those daily calculations. This is why your actual interest earned may differ slightly from the simple formula above.
How compound interest affects your monthly earnings
Compound interest means the interest you earn also earns interest. When the bank credits interest to your account at the end of the month, that interest becomes part of your balance. The next month, you earn interest on the original balance plus the interest from the previous month.
The effect is small in the first month but grows over time. If you earn $18.75 in month one and do not withdraw it, month two's interest is calculated on $5,018.75 instead of $5,000. Over a year, this compounding adds up—especially at higher APY rates.
Your bank statement shows this happening because the balance listed at the end of each month includes the interest that was just credited. If you keep the money in the account and do not touch it, you will see the balance grow slightly each month even without making new deposits.
Comparing interest across different APY rates
The APY is the only number that changes the interest you earn (aside from your balance). A difference of even 0.5% in APY can mean hundreds of dollars per year on a large balance.
| Account Balance | APY 2.0% | APY 4.5% | APY 5.0% | Yearly Difference (2.0% vs 5.0%) |
|---|---|---|---|---|
| $5,000 | $100 | $225 | $250 | $150 |
| $10,000 | $200 | $450 | $500 | $300 |
| $25,000 | $500 | $1,125 | $1,250 | $750 |
This table shows yearly interest, not monthly. To find the monthly amount, divide the yearly total by 12. The point is that shopping for a higher APY is worth your time—the difference compounds over months and years.
What happens when APY changes
Banks can change the APY on savings accounts at any time. When they do, your interest earnings change immediately for the next month's calculation. If your APY drops from 4.5% to 3.5%, your monthly interest drops proportionally.
You will see the new APY listed on your account statement or in your online banking portal. Some banks notify customers by email or mail when rates change, but you should not rely on that—check your statement or account settings yourself.
If your bank lowers the rate and you want a higher APY, you can move your money to a different bank. There is no penalty for closing a savings account and transferring funds elsewhere. Many people move their savings to banks offering higher rates when their current bank's rate falls behind.
Using a calculator to verify your earnings
Most banks provide an online savings calculator on their website. You enter your starting balance, the APY, and how long you plan to keep the money. The calculator shows you the projected interest earned and the final balance after interest compounds.
These calculators are useful for comparing accounts before you open one. They assume you do not make deposits or withdrawals, so the real number may differ if your balance changes. But they give you a realistic picture of what an account will earn over three months, one year, or five years.
You can also verify your bank's math by checking your monthly statement. The interest credited should match (or be very close to) what you calculate using the formula above, accounting for daily balance variations.
Frequently Asked Questions
Why does my statement show different interest than my calculation?
Banks calculate interest daily based on your actual balance each day, then credit the total at month's end. If you deposited or withdrew money during the month, your average balance was different from what you used in your calculation. This is normal and expected.
Does interest get taxed?
Yes. Interest earned on savings accounts is taxable income. Your bank sends you a 1099-INT form at tax time showing the total interest you earned that year. You report this on your tax return. The amount varies by how much interest you earned and your tax bracket.
How often is interest credited to my account?
Most savings accounts credit interest monthly, though some credit it daily or quarterly. Check your account agreement or contact your bank to confirm. Monthly is the most common, which is why the monthly calculation matters for tracking your earnings.
Can I earn more interest by moving money between accounts?
No. Interest is calculated on the balance in each account separately. Moving $1,000 from one account to another does not change the total interest you earn—it just shifts which account earns it. The APY of each account determines the earnings.
What if my balance is very small—do I still earn interest?
Yes, but the amount is tiny. A $100 balance at 4.5% APY earns about $0.38 per month. Some banks have minimum balance requirements to earn interest, so check your account terms. Most online banks have no minimum.