The Basic Formula Banks Use
Banks calculate interest on your savings account using a simple multiplication: your account balance multiplied by the annual interest rate, divided by the number of days in a year, then multiplied by the number of days the money sat in the account. The formula looks like this: (Balance × Annual Rate ÷ 365) × Number of Days. If you have $10,000 in an account earning 4.5% APY and leave it untouched for 30 days, you earn roughly $37 in interest.
Most banks use daily compounding, which means they calculate interest on your balance every single day, then add that interest back into your account. The next day, they calculate interest on the new, slightly larger balance. This is why the actual interest you earn is slightly higher than a simple one-time calculation—you earn interest on your interest.
The rate you see advertised—whether it says 4.5% or 0.01%—is always the annual percentage yield, or APY. That number already accounts for compounding. You do not need to do anything to make compounding happen; it occurs automatically.
Key Takeaways
- Interest is calculated daily by multiplying your balance by the annual rate, dividing by 365, then multiplying by the number of days your money stayed in the account.
- Daily compounding means interest gets added to your balance each day, and the next day's interest is calculated on that larger amount.
- The APY shown on the account already includes the effect of compounding, so you do not need to adjust the rate yourself.
- Your actual interest earned depends on your balance, how long money stays in the account, and whether you make deposits or withdrawals during the month.
- Banks must disclose their compounding method and APY in the account agreement or on the product page.
Why Your Balance Changes During the Month
Interest accrues on your actual balance each day. If you deposit $5,000 on the first of the month and leave it there, the bank calculates interest on $5,000 for all 30 days. If you deposit $5,000 on the 15th, the bank calculates interest on $0 for the first 14 days, then on $5,000 for the remaining 16 days. Withdrawals work the same way—the moment money leaves your account, the bank stops calculating interest on it.
This is why the interest you earn in one month can differ from the next month, even if the rate stays the same. A month where you make a large deposit midway through will earn more interest than a month where you make a large withdrawal. Banks send you a statement showing the interest earned each month, and that number reflects the actual balance you held on each day.
How Compounding Increases Your Earnings Over Time
Compounding is powerful over longer periods. In month one, you earn interest on your starting balance. In month two, you earn interest on your starting balance plus the interest from month one. By month 12, you are earning interest on a balance that includes 11 months of accumulated interest. The longer money sits in the account, the more noticeable this effect becomes.
A $10,000 deposit at 4.5% APY earns about $450 in the first year if left untouched. In year two, it earns about $469, because the balance is now $10,450. The difference seems small, but over decades, compounding creates significant growth. This is why banks advertise APY rather than a simple interest rate—APY shows you the real return you will receive.
Different Compounding Schedules and How They Matter
Most savings accounts compound daily, but some older accounts or specialty products compound monthly or quarterly. The compounding schedule affects how much interest you actually earn, though the difference is usually small. An account compounding daily at 4.5% APY will earn slightly more than an account compounding monthly at the same rate, because interest gets added back into the balance more often.
Banks must disclose their compounding method in the account agreement or on the product disclosure page. If you are comparing two accounts with the same APY, the one with daily compounding will earn you a few dollars more per year on a typical balance. For very large balances or very long holding periods, the difference becomes more noticeable. Always check the disclosure document if you want to know the exact compounding schedule.
What Happens When Rates Change
Banks adjust savings account rates frequently, sometimes weekly. When a rate changes, it applies to your balance going forward—it does not retroactively change interest you already earned. If your account earns 4.5% in January and the bank drops it to 3.5% in February, you keep the 4.5% rate on the interest earned in January, and the new 3.5% rate applies starting in February.
Some accounts are linked to an index, like the Federal Reserve's benchmark rate, and adjust automatically when that index moves. Others are set by the bank and can change at any time. Your account agreement or the bank's website will tell you whether your rate is fixed or variable. High-yield savings accounts typically have variable rates that move with market conditions.
How to Check Your Interest Earnings
Your bank sends a monthly or quarterly statement showing the interest earned during that period. The statement breaks down the starting balance, deposits, withdrawals, interest earned, and ending balance. You can also log into your online banking portal and view interest earned year-to-date. At tax time, the bank sends a Form 1099-INT if you earned $10 or more in interest during the year, and you report that amount on your tax return.
If you want to calculate interest yourself before opening an account, use the bank's rate and the formula: (Balance × Annual Rate ÷ 365) × Number of Days. Most banks also provide an interest calculator on their website where you enter your balance and see an estimate of what you will earn. These calculators assume your balance stays constant, so the actual amount may vary if you make deposits or withdrawals.
Frequently Asked Questions
Does interest get added to my account automatically?
Yes. Banks calculate interest daily and add it to your balance automatically. You do not need to do anything. The interest becomes part of your account balance and starts earning interest itself the next day.
Why does my interest earnings vary month to month?
Interest is calculated on your actual daily balance. Deposits increase the balance and earn more interest; withdrawals decrease it and earn less. A month with a large deposit midway through will earn more interest than a month with a large withdrawal, even if the rate stays the same.
Is the APY the same as the interest rate?
APY includes the effect of compounding, while a simple interest rate does not. The APY is what you will actually earn. Banks advertise APY because it is the true return on your money. A 4.5% APY account will earn you more than a 4.5% simple interest rate account.
What if I withdraw money before the end of the month?
You earn interest only on the days the money was in the account. If you deposit $5,000 on the 1st and withdraw it on the 15th, you earn interest for 14 days, not 30. The interest is calculated and added to your account before the withdrawal processes.
Can a bank change my interest rate without warning?
Yes. Banks can change variable rates at any time, though they usually give notice. The new rate applies to interest earned going forward. Check your account agreement to see whether your rate is fixed or variable, and sign up for rate alerts if your bank offers them.