Banks multiply your balance by the annual rate, divide by 12, and add that amount to your account each month
Most savings accounts use simple interest, which means the bank calculates what you earn based only on the money you have on deposit, not on interest you've already earned. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 12 months. That result is what you earn that month.
For example, if you have $10,000 in an account with a 4.5% annual rate, the bank divides 4.5% by 12 to get 0.375% per month. Then it multiplies $10,000 by 0.00375, which equals $37.50. That $37.50 gets added to your account on the interest payment date—usually the last day of the month or the first day of the next month, depending on the bank.
The timing matters because banks calculate interest based on your balance on a specific day each month, often called the interest calculation date. If you deposit money after that date, it won't earn interest until the following month. If you withdraw money before that date, you lose the interest on that amount for that month.
Key Takeaways
- Monthly interest is calculated by taking your account balance, multiplying it by the annual interest rate, and dividing by 12.
- The bank uses your balance on a specific calculation date each month, so deposits made after that date earn interest starting the next month.
- Some banks compound interest daily or weekly instead of monthly, which means you earn small amounts of interest on your interest.
- Your actual monthly earnings depend on both the interest rate and how much money you keep in the account throughout the month.
How the calculation date affects what you earn
Banks don't use your balance on every single day of the month. Instead, they pick one specific date—often the last day of the month or the first day—and use whatever balance you have on that day to calculate that month's interest. This is called the interest calculation date or statement date.
If you deposit $5,000 on the 28th of the month and the bank's calculation date is the 30th, you'll earn interest on that $5,000 for that month. But if you deposit it on the 1st of the next month and the calculation date was the 30th of the previous month, you won't earn interest on it until the following month. Check your account agreement or call your bank to find out when their calculation date is.
Withdrawals work the same way. Money you take out before the calculation date reduces what you earn that month. Money you withdraw after the calculation date has already been counted, so you've already earned interest on it.
Compounding interest versus simple interest
Most savings accounts use simple interest, but some use compound interest, which means you earn interest on your interest. The difference grows over time, especially if you leave money in the account for years.
With simple interest, you earn the same amount each month as long as your balance stays the same. With compound interest, each month's interest gets added to your balance, and next month you earn interest on that larger amount. Banks that compound interest may do it daily, weekly, or monthly. Daily compounding earns you slightly more than weekly, which earns slightly more than monthly.
The difference is small in the short term. On $10,000 at 4.5% annual interest, simple interest earns $37.50 per month. With daily compounding, you'd earn roughly $37.52 per month—only a couple of cents more. But over five years, that small difference adds up. Check your account disclosure or ask your bank whether they compound interest and how often.
Why your actual earnings may differ from the advertised rate
Banks advertise an Annual Percentage Yield (APY), which is the rate you see in ads and online comparisons. This is the rate used to calculate your monthly interest. However, your actual earnings depend on how much money you keep in the account and for how long.
If you open an account with $1,000 and the advertised rate is 4.5% APY, you won't earn 4.5% of $1,000 in your first year. You'll earn 4.5% only if you keep that full amount in the account for the entire year. If you withdraw $500 after three months, you'll earn less because your balance was lower for nine months of the year.
Interest rates also change. Banks can lower the rate they pay on savings accounts at any time, though federal rules require them to notify you first. If your rate drops from 4.5% to 3.5%, your monthly earnings will drop too. Some accounts offer a may provide rate for a set period; others change rates whenever the bank decides to.
How to find your interest calculation date and rate
Your bank should provide this information in your account agreement, which you can usually find on the bank's website or request by phone. Look for terms like "interest calculation date," "statement date," or "compounding frequency." Your monthly statement also shows the interest you earned that month, which you can use to verify the calculation.
To check the math yourself, find your average balance for the month (add up your balance on each day and divide by the number of days), multiply by the annual rate, and divide by 12. The result should be close to what appears on your statement. It may not be exact because banks sometimes use different methods to calculate average balance, but it should be within a few cents.
If your statement shows interest that seems too low, contact your bank. Errors happen, and the bank can review the calculation. Keep in mind that if your balance was lower than you expected during the calculation period, that explains lower earnings—the rate itself may not have changed.
How deposits and withdrawals mid-month affect your earnings
The timing of deposits and withdrawals matters because of when the calculation date falls. If you deposit money on the 15th and the calculation date is the 30th, that money earns interest for that month. But if you deposit it on the 1st of the next month after the 30th calculation date has passed, you wait until the following month to start earning.
Some banks use an average daily balance method instead of a single calculation date. With this method, they add up your balance for each day of the month and divide by the number of days. This is more generous because money you deposit partway through the month still earns some interest that month. Ask your bank which method they use.
Timing withdrawals is less important for earnings, since you've already earned interest on money by the time you take it out. However, if you withdraw money right before the calculation date, you'll earn less that month. If you need to make a large withdrawal, doing it after the calculation date means you don't lose a month's interest on that amount.
Frequently Asked Questions
Does interest post to my account on the same day every month?
Most banks post interest on the last day of the month or the first day of the next month, but the exact date varies by bank. Some post on the 15th. Check your account agreement or recent statements to see the pattern. The posting date may be different from the calculation date—interest might be calculated on the 30th but posted on the 1st.
What happens to my interest if I close my account mid-month?
You keep any interest that was already posted to your account before you closed it. Interest that hasn't been posted yet depends on when you close the account relative to the calculation date. If you close before the calculation date, you may not earn interest for that partial month. Ask your bank about their policy before closing.
Can a bank change my interest rate without telling me?
No. Federal law requires banks to notify you before lowering the rate on your savings account. The notice must come before the rate change takes effect. However, banks can raise rates without advance notice. If your rate drops, you'll see the lower amount reflected in your next month's interest payment.
Why does my statement show interest different from what I calculated?
Banks use different methods to calculate average balance, and they may round differently than you do. Some use the balance on the last day of the month; others average daily balances. A difference of a few cents is normal. If the difference is larger, contact your bank to ask which method they use and verify the calculation.
Is compound interest better than simple interest for a savings account?
Yes, compound interest earns you slightly more money over time. However, the difference is small on most savings accounts, especially over short periods. On $10,000 at 4.5% for one year, you'd earn roughly $450 with either method. The bigger factor is finding an account with a higher interest rate, which matters far more than whether interest compounds daily or monthly.